Every family is different, but actually, according to research, the closer the kids are to each other; the more likely it is for them to be different.
It is not a formal decision the child makes, but early on they might start seeing that one is recognized say by reading well at an early age, then they might try something different.
Kids also will tend to compete more with each other if they are the same gender.
My brother is only 16 months older than I am. WE fit the typical pattern in that we boith have many "older" characteristics; but I also have some Sandwich traits that he doesn't have.
Again, it doesn't mean anything in terms of individual families/kids; these are just tendencies.
