Unless I'm misunderstanding how this works, the same amount of STEEM should be available in the reward fund so the payouts should never increase or decrease when viewed in terms of STEEM.

When viewed in terms of SBD obviously they will fluctuate based on the market price of STEEM, so I'm guessing that the lower gross payout value for Sept vs August was just due to a lower STEEM price. Let me know if I'm saying something incorrect here.