Because 25% of your post value goes to curators (the accounts that upvoted your post) Then what remains is split in SBD (Steem Dollars) and SP (steem power)
PS I Agree that this is super confusing !!! They should fix it and show us independently what exactly the poster will get and what the curators before the pay out's happen... That should be very easy to implement... When mouse over : show what curators get, when not, show what the poster gets!
PS2 I opened an issue on github since I think it really very important to change ASAP !
https://github.com/steemit/steem/issues/212