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.
Huh, now that you mention it, after rock climbing, I similarly got the least callused on my index finger. Even the base of my index near the palm is much less worn than the rest of the non-thumb fingers