How rewards are calculated on the Steem blockchain
The main confusion point is the "$" displayed by default near the rewards by Steemit (and busy, and eSteem). This "$" is not a USD. It has nothing to do with it. It's also not a SBD. So, what is it? It's what I'l call from now on a "Steem Token Unit", or STU. This is different from SBD and STEEM as currencies, and it's actually the expression of the underlying value storage in the STEEM blockchain, which goes under the name vests.

So, the potential reward you see near your posts is actually expressed in STU.

Example: you see near your post $100. In order to understand how much you will get, we will talk form now on in STU whenever we refer to the numbers the Steemit UI is showing. So you actually see near your post 100 STU.

Out of this, you give away 25% from your STU to curators.

You remain with 75 STU.

This amount is split 50 / 50, with the following algorithm:

the first half is given in SBD, in the exact amount shown by division. In our case, half of 75 is 37.5. So you get half of your rewards as 37.5 SBD
the second half is given in Steem Power, at the current feed price (the internal Steemit price for STEEM) in USD. So you get $37.5 worth of Steem Power. If the current feed price of STEEM is, let's say $6, you get $37.5 / $6 = 6.25 Steem Power.
Please note the difference: you get SBD directly as half of your rewards, but you get your Steem Power at the current market price of STEEM, not in absolute numbers.

You get half as SBD, and half as usd equivalent value expressed in SP, at the current price of SP in USD.

The end result for something that displays 100 STU near your post:

37.5 SBD
6.25 Steem Power
Let's see how much this means in fiat at the following prices: STEEM = $6, SBD = $8.

37.5 SBD x 8 = $300
6.25 Steem Power x 6 = $37.5.
So $100 displayed rewards, are actually equal = $337.5 real USD.