Don't underestimate the power of reflex action. Bandits may be relatively weak, but they're still warriors - how else would they be able to hold their positions? It seems that, even if you don't expect it, an arrow out of the dark would be more than enough to set off your internal alarm bells and make you step aside. Of course, staying in place after that is likely bugged, but you could always put that down as them being shocked enough by the first arrow to not notice the second.
This is, of course, assuming they're facing you. If they sidestep arrows flying at them from the back, that IS broken.
Ha, maybe but have you ever been shot with an arrow? They move kinda fast, about 115MPH for an arrow fired from a longbow. Having said that, an enemy who is aware they're being fired at may pre-emptively duck and weave to avoid being hit. I think that's fine, the issue I've experienced is the same as many others here - from a sneaking position, loosing an arrow at a completely unaware enemy, only for them to get up and dodge it.
At first I thought this was just bad luck, or a coincidence they happened to move, but I've tested it, even on sitting targets who mysteriously get up the second an arrow is let loose. Basically the game is flagging them as alerted the second your bow is fired, instead of when a hit actually impacts either them or a nearby surface. At short range this is barely noticeable, but at longer ranges (and let's face it if you're a sneak-based character then you'll be firing that thing from as far away as possible lest they close in and smash your face with a warhammer) the effect is pretty ridiculous; Somehow a load of bandits gain either incredible luck or matrix-esque reactions.