I frankly don't know. I went to a "gifted school" as a 1st grader that required you to pass an IQ test to get in and have >150 on their chart. This was in the early 1980s. At least half the kids were what we'd now call "on the spectrum". A couple of them were already performing at college level in math by the time they were 11. One of them went straight to UCLA when he was 13. I was programming in HyperCard at the time and could simply not fucking grok the math that the kid at the desk next to me was just absolutely smashing. I was inherently intelligent and I was raised with the best possible chance of getting to that level of competence and ability, but I ended up being an art school dropout who programmed a bitcoin casino and still can't deal with multivariable math unless I write a block of logic for it. I have no idea how or why that kid (Eric Kim was his name) was so much more brilliant than me. We were in the same exact math class with a really great teacher who also happened to teach the afterschool "programming club" in the Mac SE lab, in HyperTalk.
One kid just groks the math. One kid groks the fun(){} ...in a perfect world, those brains should just complement each other, I guess.
Different mental starting points and a healthy dose of randomness and luck. To me, learning maths always felt like playing puzzle games. The more you play different games, the more you get used to the "secrets" behind the puzzles and the easier your brain makes successful connections. But when you're learning calculus, you barely have time to make those connections. Give the same starting point and same experiences, brains will make different connections and learn things a little different.
I have a similar story as noduerme, but for me it was interest and laziness. I got by in all my pre-college math just by listening in class. But, if I couldn't connect it to something I found fun (like taking apart and putting back together my NES for the 100th time, or playing some sport), I just didn't do anything beyond the minimum. Then college came around, and I got an F in my first math course. I retook the course and found a teacher who connected most topics to gambling and/or business and I was hooked. I managed to take that teacher for almost all the math I needed for my CS degree :)
