It's because of the printing, which goes on according to the inflation parameters already set on in the blockchain. Printing went on even when there was no interaction on the platform, because of the HF 20 bugs, so STEEM was printed, but no one was there to "distribute" it. The increase is coming - mainly - from that source. Also, because some curation rewards are redistributed to the rewards pool if an author self-votes in the first 15 minutes - in which case the rewards pool doesn't decrease as fast as before.
As the first payments after the 7 days window are approaching, we will see a decrease in the rewards pool.
Does this makes sense?
Aha! Very much so. Thank you @dragosroua!!!