I'm guessing that they also have a lotof Spaarti clones rather than Kamino bred-clones, leading to poor accuracy averages. The stormtroopers have a lot of regular civilians, who aren't born to fight like the clones.
Seriously, I've not even doing this with the stormie-can't-hit-stuff stereotype. Why Clone Troopers are better than Stormtroopers:
