There can be a great variability among members of the same types, even when accounting for subtype. There are two different theories for explaining these differences, wings and trifix.
Wings
The wing theory is one that has been developed primarily by Riso & Hudson. The idea behind the wing theory is that types are influenced on a continuum by the types neighboring them. For example, fours can be understood as the midpoint between three and five, but individuals who type as four may vary considerably in the degree to which the neighboring type will influence them. Some fours may feel more strongly influenced by five, which would make their type a 4w5, whereas other fours may feel more strongly influenced by threes, thus they are better categorized as a 4w3.
Some enneagram theorists have embraced the wing theory, developing it further. Both Tom Condon and Judith Searle have their own interpretations of how the type/wing combo interacts. Other theorists, like Sandra Maitri, depart from the one-wing approach, preferring to see the types as equally influenced by neighboring types.
From the standpoint of a continuum, the wing theory makes sense. People will likely fall in various spots on the continuum. For example, a seven could be so close to their eight wing that it's hard for them to decide whether they're a 7w8 or 8w7. Some sevens may be much closer to six, and see very little 8 influence in their personality. Others might find it difficult to discern whether they have an 8 wing or a 6 wing.
The problem with the wing theory is that it's still relatively undeveloped, with limited information on how the type/wing combos intersect. Riso & Hudson have the most information, writing roughly 2-4 pages on each type/wing combo in personality types. However, a lot of this information is based on stereotypes--5w4s are drawn to the humanities, 5w6s to science, 4w5s are bohemian and philosophical, not wanting to "sell out," 4w3s are commercial artists. 6w5s are introverted, independent and political. 6w7s are dependent, sociable but not particularly political, etcetera. Much of this information is unhelpful, partly because it doesn't really combine the key issues of each type, but rather relies on stereotypes. The bohemian, "will never sell out" artist at 20 may find themselves driven to promote their work and make political connections at 30. Or, a person may be self-promoting in some aspects of their life, while private and creating solely for themselves in other areas of their lives. The "stuck in the laboratory" neuroscientist may go home and read Camus. The six political activist might still be outgoing and relationship-oriented. Other theorists have explored the way types and wings interact, and come up with other descriptors, although for the most part, these are brief and still in need in further development. For this reason, the wing theory has come under some criticism, with considerable debate about whether it's valid or not.
Trifix
Even less seems to be known about the trifix theory. This was put forth by Oscar Ichazo. He rejects the idea of wings, saying rather that we have a type, with the corresponding type issues, in each of the triad. Basically, all nine types fall within three triads. The heart triad--types 2,3 and 4--have issues with shame and self-image. The head types (5,6 and 7) have issues with anxiety and authority. The instinctual types (8, 9 and 1) have issues with anger and autonomy. So, while one enneagram type may be dominant in your personality, certain life situations cause the other types in your trifix to manifest. For example, if your primary type is a four, but you deal with anxiety and authority issues through seven tactics (distracting yourself from anxiety, tending to take an egalitarian approach, seeing yourself as neither subservient to authority nor as an authority to others) then you would have seven in your trifix. Similarly, if you deal with anxiety through developing expertise and being in a constant state of preparation, you probably have 5 in your trifix. If you have an ambivalent relationship to authority, where you're either constantly trying to curry favor or rebel against authority out of fear of being controlled, you probably have six in your trifix.
For some people, the trifix is really easy to figure out. For example, if your three highest scores on the RHETI are 7,8 and 3, and all three resonate strongly with you, you probably have some form of 7-8-3 in your trifix. For others, it may be less clear, for example if your three highest scores are 4, 5 and 7, it's going to take a bit of reading and introspection to figure out which head type fits. In general, look at the domains of each triad and ask yourself how you deal with those situations when it comes up, to figure out which types are in your trifix. It can also help to look along subtype lines. So, if you're a sx 7 and can't decide if you have 2 or 3 in your trifix, look at the descriptions of sx 2 and sx3, and see which is more pronounced in your personality.
One of the criticisms of the trifix is that it tries to make the enneagram encompass every aspect of the personality. I generally find this to be unfounded. First, the enneagram is a triadic theory. Three triads containing three types within it, then three subtypes that belong to each type, and each type having the directions of integration and disintegration, that also operates on a principle of three. There's no reason to think that we wouldn't each have a dominant type in each trifix.
Furthermore, even when you belong to a given triad, life isn't restricted to one domain. We all have to deal with shame and identity, anger and autonomy and anxiety and authority based on what situations we're dealing with. While the type in our triad may be our baseline, and those issues may affect us most prominently, there will still be variability within our given type of how we deal with the issues that crop up in the other two triads.
Additionally, complexity can be used as a good starting point for self-exploration, and simply developing a more comprehensive system isn't the same thing as trying to have every aspect of the self fit into the enneagram. Even accounting for trifix, type and subtype, there will still be a lot of variability among people that have the exact same combos as we do. Others come from different home environments, have lived in different parts of the world, have different interests, and have been shaped by different relationships, successes and setbacks. What this will do is give a clearer picture of what defense mechanisms we bring to those differing experiences, and how we're likely to react, respond and process what has happened, in a way that can hopefully give us insight into ourselves and others in our lives. From that perspective, I don't see how an added layer of complexity could possibly be a bad thing.
Either/Or?
Some people believe in an either/or approach to the wing/trifix debate, where they reject one in lieu of the other. In general, I say, whichever works best for you. If you don't like either theory or it doesn't make sense to you, then perhaps you don't want to incorporate that into your own self-exploration. Others might find both to be helpful. I personally prefer adopting both a wing and trifix approach. In general, I encourage people to look into both theories, decide for themselves which works, while being open-minded to the pros and cons of each.