A significant majority of the stormtroopers in the timeframe of the original trilogy were not clones and were just recruited. The clones were abruptly phased out of service due to a clone uprising on kamino and only a single legion of stormtroopers remained clones of jango fett, hence the recruits had extremely bad aim compared to men who had been specially bred for combat.

They're not clones by then, for one. For two, they're wearing armor with much less practicality for their accuracy, and using a blaster that was designed as more of a storm weapon. There's also speculation that, when they're having to shoot non-uniformed, very human combatants, they're unable to bring themselves to actually shoot to kill. They're aiming wide to avoid having that guilt on them.