I have a tiny spot of quite thick callouses on the side of the top of my middle finger. Clearly from some regularly using that spot for something. I have no idea what it could be though.
* puzzled *
I think I have the same thing, it's from holding pens.
That's one of the reasons I was puzzled, I did not use a pen enough to get callouses, and certainly not a spot that thick.
So, still puzzled. Most of the callouses is gone now.