Hand
size, too. I have big fuckin' hands, and I can comfortably manage chords in eg Chopin pieces that my (small, female) piano teachers had to roll. Conversely, they could more easily do some of the arpeggiation that I'd almost literally fat-finger at non-practice tempo.
I'll also add my anecdata to the parent's thoughts on quantization of fingerings to tempos. I like ragtime, and I've grown to notice that playing it properly (that is, slowly, as Joplin is always going on about in the margins) often requires what is essentially more difficult fingering than that which is required to play it quickly (that is, the !!FUN!! way). Someone in another subthread mentioned having to physically model the arm and hand. I think that's essentially correct.
>You must first access the corpus of data on fingerings. AFAICT that is an oral history passed among piano teachers.
At the risk of a vague digression, I'd also like to point out the difficulty the parent had in extracting exactly what their hand was doing outside of the context of "sitting in front of a piano, playing the notes in question". The whole point of fingerings being added to a difficult section -- whether by the publisher or the performer -- is to aid the speedy automatization of that difficult section, with the aim of converting it into an uncontroversially straightforward section, like those in the rest of the piece that don't need fingerings. The best piano teacher I ever had, when working out fingerings for a difficult unlabelled section, would play it slowly a few times while looking at, and thinking about, her finger position. Then, she'd try to play it at as close to full speed as she could, and observe what her hand was doing. That is, leverage the automatization that fingerings are supposed to supplement. It's not just piano teacher oral histories one should ought to digitize, it's also piano teacher premotor cortices!