In the old days you were able to disable xp gain and keep racking up kills in the dungeons. So you could get to lbl 25, lock xp gain and just stay at fathom crypts forever and get 10s of thousands of kills. Its the only reason how low lvls have high kill count.

Its not possible to do this now because the devs has realised it made an unfair effect on the leaderboards and they have stopped kill count rising when xp is not gained. Same as you dont get kill count by going to a lower lvled dungeon.