Can any one tell what is the reason for uneven level structure??

Question

1st 15 levels are within 20,000xp
Level-16 , 20,000xp level 17 1,00,000xp
#16-#17~ difference 80,000xp
#17-#18~ difference 4,00,000xp
#18-#19~ difference 2,50,000xp
#19-#20~ difference 2,50,000xp

Answer 1

I would say to make it more difficult as one progresses in level.