Excellent explanation, @ilyastarar!
I would just like to expand a bit on this:
A quick calculation is needed to clarify rewards distribution briefly. Suppose your post shows $100 reward. Upon payout, you (author) will only get 75 % of it (that too half in SBD and half in SP). Rest of the 25 % is converted to SP and distributed among people who upvoted your post (curators). For $37.5 (half of 75 %), you get exactly 37.5 SBD. The other $37.5 are converted into SP as per current USD price of Steem (you get as SP whatever number of Steem can be bought with 37.5 USD).
It is important to note that as long as SBD price is above 1 USD, it is much wiser to use 50/50 payouts instead of 100% Steem Power payouts. This is true because, while Steem Power received does gets divided by its current price, you always get SBDs as their price was 1 USD - they don't get divided by their current price.
Cheers! : )