Experience bonus for low amount of unidle players
Unidle experience bonus used to be a feature that gave you +25% more exp from kills if there was low enough unidle mortals in the game.
"Because of the low amount of unidle players you receive some additional experience for your kill."
It is a known issue that many NPC articles may have too high exp value if the article was added before the aforementioned tune. Fixing the values is wearisome manual work, so you probably should remember to take this into account for a quite a long time still if you're planning to kill an NPC which seems like "too good exp"...