I wondered about this, until it hit me. Imagine two bearings at right angles to each other, which only offer movement in one plane. If the two bearing are coincident, the effective length will be the same. But if the bearing responsible for vertical motion is a distance further away from the spindle than the bearing that is responsible for horizontal motion, the two effective lengths will indeed be different. Unless I'm missing something...
Yes I eventually thought about this too last night, but what does this have to do with how the cutting head cuts and why?