For +experience on items, it seems that the full amount of experience isn't rewarded per kill; instead, it seems based on the monster killed. On the character sheet, bonus experience is listed as "up to" the combined amount of +experience from gear.
What determines how much bonus experience is granted per monster?
The monster level versus your level determines how much xp you get, and whether or not you get any bonus xp.
If we consult this handy chart you will notice at monster levels more than 5 levels below you, you get no +xp/kill bonuses.
I don't know if you would only get 23 bonus xp for killing a monster 4 levels below you though.
I've done some quick and dirty tests, and I'm pretty sure that the bonus XP is added onto the monster's base XP before any multiplayer or level based calculations are applied.
This means that, for instance, you can't stack +500 XP on a nightmare-level character then one-shot a few thousand normal-mode monsters for easy XP.
