Rewards are calculated to drain the pool of newly created Steem each day, lagging 7 days. Just after HF19 there was an accumulation of Steem beyond that, due to whales participating in an experiment and not voting. We have drained that excess, and Steem being produced each day is being consumed each day, if I understand correctly (I could well be wrong, as these are but recollections of passing mentions, rather than study).

This smaller rewards pool, consisting only of what Steem is mined each day, also contributes to lower rewards, but the impact is nowhere near as great as the decline in the price of Steem.