In my experience, NPCs have fast turn speed but if you are fast enough, you CAN get beside or behind them. One of the other things that you may have noticed is that NPCs (and you) have a large "combat hit cone angle" - by default, you can hit people 35 degrees to the left or right of your crosshair heading, which is quite large.
I made a mod that addresses these issues a little. You can find it here: http://tesnexus.com/downloads/file.php?id=28318
I'm working on a new version of it too, by the way. No promises on release date though.