Hilariously, I anecdotally also can relate to this. I'm a competitive powerlifter and the calluses on my hands and fingers are most pronounced on the middle, ring, and pinky - with very little on the index.
I do pullups sometimes and I also observed this. I wonder if it is related to how index finger is not really in-line with the arm. When I look at my hand I can see that if U would draw extension lines from edges of my forearm they would contain middle, ring and pinky fingers, but not really the index finger.