For example the strength of your introversion isn't purely "personal", much less a predetermined biological fact. It is of course very cultural. Anglo-Saxon cultures encourage introverted behaviour. A lot of people who think it's completely natural to be as introverted as they are, if they had grown up in a culture where you constantly have large families visiting you all the time, to give just one example, would not be as introverted as they are, nor think it natural to be.
Likewise with many things. For example I think its "natural" to be as T as I am, but obviosuly being a male, this has been socially constructed to a certain extent.
In short - MBTI explains the functions but the end result is hugely varied according to culture.