Me & my sister grew up, like you say, in the same family, same parents, same pressure. We largely went to the same schools, even. She probably acceled further, academically, than I did. (She probably went to the more prestigious college, her GPA/SAT/etc. were better, she earned a doctorate, while I got a BS…)
Without pointing to a vague notion of "we're different people", I think there's a few key things that were different, despite everything that was the same:
* She's the second child, I'm the first: there were some things in my education that my mother literally said "we are fixing that for her". (And I should note that I don't resent this: my mother was clearly doing the best she could with the information she had — and because she loved us. But she had more information during Round 2.)
* Education is a finite resource: in my home state, whether I got into a decent school (i.e., a magnet school) was dependent on the literal roll of a dice. (Literally literally. I.e., list of names goes in, gets shuffled, top n go to good school & gets educated, bottom m talent gets wasted.) In the worst case I was 5th? 6th? from the bottom of a several hundred person long wait list. She got in. She got a year in the magnet system that I didn't (I got entry a little over a year later). That missing year was an enormous detriment to my education and growth; it was such a clear detriment my parents were contemplating whether they could afford a Catholic private school (we're not Catholic) or simple home-schooling. Had they had the gift of clairvoyance, I think they would have the moment I was denied.
* Almost certainly the gift of a computer got me interested in CS. She didn't get one, and her interests are different. (She's still STEM, likely due to our parents.)