Out of curiosity I watched myself playing. I noticed that because of my long fingers and the fact that my thumb is much shorter than the rest, my second and third fingers tend to turn sideways when playing between the black keys. Maybe that's why I have no difficulty there. That also answers an old question of mine as to why I always had difficulty in playing some of the fast twiddly bits. I'm not striking the keys as accurately as I should. I'm surprised I didn't notice it before. Another interesting fact - the space between the C# and D# keys on my keyboards is a lot bigger than on the piano which has more even spacing - as you also discovered while measuring.

Bryan