Velocity difference between piano roll and drum map....

Why is it that when editing velocities in normal piano view, you can select any velocity between 1 and 127 but when you use a drum map, you can only select every third or fourth velocity?

EDIT: I guess I should clarify that I'm referring to the velocity audition function. When using a drum map, it skips three or four velocities at a time. In normal Piano Roll View, every velocity is available.

Any ideas?

Thanks