If a mob has been alive in the world for a long time, it starts accumulating “bonus xp”. A separate pool of xp that increases the longer the mob is alive. The idea is that it gives incentive for players to go kill all sorts of stuff because there is a chance that the mob might yield bonus xp.
But why is this only limited to xp? Why is magic find or rare drops not heavily increased on mobs that are rarely killed? If it was really worthwhile people would literally kill everything (because you cant see if the mob has been a long time.. you just need to kill).
the idea is that you do the opposite of DR. It will be IR. Increased Returns. players are heavily and significantly rewarded for killing all animals in the knicks and crannies of the world that has not been killed for a while. it spreads people away from the heavy farming segregation.