Well, it's all about dynamics... in every way.
You obviously know signs, and perhaps birth charts. Those define your personality at birth and what your personality might be if you lock yourself up somewhere you will never experience any sort of interaction and never learn anything.
As you probably well know, that would be impossible, which also suggests why people of a certain sign don't act like that sign per se, or people with exactly the same birth chart can still act differently. We are all raised in different environments, with different people and different personalities. For example: someone with mercury in Sagittarius is very likely to say what is on their minds without thinking it, and other people's reactions to it, over. This sometimes causes serious fights if in conversation with the wrong person. When a Sagittarius mercury says something offensive to someone without realising it (which often happens; they just don't realise they can be offensive) and that person is either too scared or too "civilised" to say something about it, then the Sagittarius mercury will never learn of the effect of his words. When that same Sagittarius mercury says the same thing to someone who WILL tell him, and very promptly, that they are offended then he will know the effect of his words. And when it turns out in an argument, I would say it's a lesson learned. I have no doubt that the Sagittarius mercury won't repeat something like that again if they are told about the effect of their words. Just a simple example like that can make two people with exactly the same placement different, just from the different experiences.
Another example could be of two people born around the same time and have exactly the same chart, but for the sake of argument, one is born in Western Europe, the other is born in South Africa. Both areas have different cultures, and if both children are raised in their respective country and never come into contact with another culture, they will only be influenced by the culture they are in, despite their personality. In every case, your personality is also forged into what it is now by the culture you are raised in as well as the environment in as much as it is possible to forge your personality (if you're naturally kind hearted, there is no cultural or environmental influence that can make you less kind hearted, only on the surface - but that is called cynical; if you naturally don't have a knack for languages, it doesn't matter how many languages are spoken in your country of origin). So, those two people will still have the personality that they are born with, but with clear influences of the culture they are born into.
Hope that makes sense. Just remember that there is more to astrology than just the sun signs. Feel free to explore.