1. I don't wear a ring on that finger.
2. I don't spend enough time behind the steering wheel to have gotten it from driving.
3. I'm not athletic and don't lift weights.
4. I don't grip a pen or pencil in such a way to cause a callus on that part of my hand.
So, through process of elimination, here's what I have deteremined: The only other repetitious activity that could possibly make a callus there is . . .
MY COMPUTER MOUSE!
Haven't figured out yet what to do about this phenomenon. But I guess it says a lot about how much time I spend at my computer.