What is the formula for additional experience on items?
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.