Once you get past level 3, your XP tends to be way over the level 4/5 limit. If you've then got maybe 10/20 nodes to write for the next level, you have got less incentive to write good and/or interesting nodes. You've got XP to burn, so you stop caring. (even subconsciously)
This is where I really feel the need for some good graphing capabilities built into E2. You could plot a bar graph showing XP per node, then see if your node quality really does decrease as you near the next level.
Ironic that I now need 2 more nodes until level 4