A long time ago they changed the code for mob death to cap to a certain percentage at every level and every level will be different. I have worked myself out of a 350k xp hole before all these changes were made. They put the cap in because of people taking 30hours just to work out of a hole like that and to make it more player friendly to new players at that time. This was like 10 years ago or more that this change was implemented. If you were not around at that time be thankful. Those huge xp holes SUCKED.