D&D 5E XP Table Strangeness

[gideonpepys]

It appears to cost more XP to advance from 9th to 10th level (16000) and from 10th to 11th (21000) than it does from 11th to 12th (15000) and from 12th to 13th, or 13th to 14th (20000).

Is this an error?

If the justification is that it slows down progress between tiers (or some such), why does it not occur elsewhere on the table?

 

[MortalPlague]

I'm not sure where they said it, but the designers said the XP progression is deliberately not linear. The game puts up a few speedbumps at levels where the game dramatically changes once you pass them.

Of course, as a DM, you're free to change that to suit your needs.