As far as I know it's a bit more complex: 1SBD was made to have a constant value of 1USD, but that didn't work out (it's about 4USD : 1 SBD). The rewards are shown under the assumption that 1USD = 1SBD.
An example: I'm getting payed 50% in steempower and 50% in SBD. My last payout was shown as 0.026USD so I got 0.013SBD and the remaining 0.013USD were equivalent to the 0.0026SP that I got.
I hope that helps!