Also, consider the time/places we see them with terrible aim. They're usually in places that would maintain more cursory guards, rather than elite forces. Death Star was a mobile fortress, which was believed to be impregnable. Perhaps those were more a security detail than a combat unit. Also explains their more defensive/panicky strategy. Bespin. Again, a fairly defensive/non-combat specialized grouping, more for appearance than anything. They had Fett and Vader to deal with any trouble. On the other hand, the group on the Tantive IV were fully combat-specialized, better marksmen, etc. That was a full-fledged raid. And the only reason Endor wasn't going as well was the enemy had camo, subterfuge, and were severely underestimated by the troopers.