OP,
It could be because some tables specifically recommend
(due to alignment) either a 9" arm, or 10" arm.
I've used 9, 10, & 12" arms (different manufacturers) but was waiting for someone to chime in who used various lengths of the same arm on the same table, (which noromance did) to note their differences.
FWIW, Of the 9, 10, & 12" arms I've used, I prefer the 12" for the reasons noromance stated, but it's really apples/oranges due to the fact that they are not the same arm (arm geometry will make more of a difference to SQ than arm length). Whatever length arm you decide upon, being a perfectionist with cartridge alignment is very important to overall SQ.