if the spoke tention is even then the frame is flexing, just look at the chain close next time.
fact- i just had a bowl of cornflakes.
lets get away from the simple idea that the pads act as a vice holding a lump of metal, think about how brakes on forks with rear mounted brakes are so much better.
yes less flex= more force, if you think about the sidwall of the rim and how thin it is, its not going to have the same amount of rididity on the surface as its radial
next thing is how comes the brake is different depending on wheel ratation?
like loads of people have said its all down to the amount of flex, and there is defenatly and optimuim amount, i suppose this is down to frame materials rim,and pads etc..
when the brake is pulled the whole stay and pad are pretty much deformed, the pads acting like a wedge thats why even with the pads hitting perfect they still wear in a funny shape.
this is also why forks with rear mounts are better, the wheel forces the pads forwards trying to jam them in..
the forces have to go some where.