Because your hand doesn't apply even pressure across the entire width, evenly, so things can go a little lopsided, quickly.
In theory, you're right of course. It shouldn't matter. But experience shows it does.
ah, of course !
to be honest, I took that aspect of the procedure as given....
Ive re-raduised two fingerboards to date - not a wealth of knowledge I know but... they have both been spot-on !
as long as even pressure is applied across the sanding block and long even strokes are taken, everything should be cool !
or am I missing something ? - I always check my work with a radius gauge and so far - no problems...