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Slide a centrelock rotor onto a hub spline and notice how much rotational movement there is. When you consider you have 2 companies making 2 different parts (one for rotor, one for hub) it doesn't take much of a machine tolerance difference to create some space to allow movement. Yes I agree that pads can move in some calipers .
scultura-
Put a 6 bolt rotor on a hub with the bolts in only a few turns of the threads and see how much rotational movement there is.
Hint: it's probably more than centerlock splines. The point is that they're both held firmly against the hub by the lateral clamping of a lockring (or 6) -
You also only ever brake in one direction, so once the disc sits against the splines, they never move back unless you pull on the bike while braking.
I've actually noticed that the cheap shimano hubs have the best fit with centrelock rotors. I think because they don't post-machine the hubs after painting. -
There's small spacers in some centerlock lockrings. I normally have a few of these for spacing discs, can be used for wobbly rotors too.
PhilDAS
chiroshi
Netakure
How do 6 bolt make a difference? Surely they are both securely clamped to the hub?