Could be: 1) market price of STEEM dropping (this is lagged 3.5-7 days), therefore less SBD distributed and also could be 2) rewards more spread out so the top 50 getting a smaller piece.
EDIT: Correction. @masteryoda informs me that the totals include all payouts so top 50 getting a smaller piece would not make a difference.
I think it's due to the price drop, as the total SBD in the report is for all the payouts, not only the top 50.