you may want to look at http://www.gamesas.com/index.php?/topic/1097214-gameplay-mechanics-anolysis/. it is stated there, the awareness-check is performed every five seconds and there is no multiplier for NPC's disposition towards the player. if you are fast enough and quickly out of range, you wouldn't be noticed by the enemy.
i've also the feeling that lower framerates let them also react slower, maybe this script is skipped by accident.
I wish that it was as simple as a framerate issue ive experienced this on a new rig.
This may explain many of the cases, but what about when you walk up to a bandit in a cave, have a chat, loot the chest behind them and then when you are walking away about 8-10 secs later the npc attacks.
As with my issue with my skeleton which was a 20 sec stroll around the room and going right up to it and walking past it. It is on the verge of gamebreaking. Maybe Hrnchamd will find something more concerning the creatures.
Some occasions might be because of statics which is partially hiding the pc from the creature of npc. Though this is simply speculations. And wouldn't explain the issues stated above.