An overstock of coins.
you really think they will directly sell 100% of the coins they get?!
imho they decided to jump in the bitcoin train not only to expand their marketshare, but also to make even more profit hodling some of it.. I mean, isnt that all about profit after all?
edit: Coinbase then, but same principle applies.
But consider GoxBux... Discounted (currently around 10% looks like) because it's hard to do anything with them once you have them. By adding more easy exits to Bitcoin ownership, it makes them a more attractive proposition to own in an inverse manner.