Think of it as instead of 50/50, where half of the amount you see is going to SBD and the other half converts to SP via feed price, it's more like 49.9/51.1 or something. With the difference being that extra 0.1 is STEEM instead of SP.

The short of it is, with SBD being worth more than a dollar, the USD value of the payout is effectively lower than before, since less of our reward is in SBD.