I don't know if this is a bug or glitch or not, but on my non-rooted Kindle Fire(not hd) the xp from level 15 to 16 is 1600, 16 to 17 is 1600, and 17 to 18 is 1600. I am currently level 17 so idk if its the same from 18 to 19.
Let me know if you are experiencing this issue as well.
~a~