Look my profile....is the Same for me. :)
It happens because more (and stronger) players plays Blitz....and there are few and weakers players for classical.
So if you are (like me) a "medium" player...you can reach easly 1700-1800 in classical..but only 1500-1600 in blitz because the players pool is bigger and stronger.
Infact , with 1500 in blitz, i'm considerated an "medium" player with a 55 percentile