Thank you @chavacrypto for the comment. The rewards are changing due to the value of STEEM and SBD with a delay. Seems like the SP is consistent and that the adjustment is only made on liquid STEEM and SBD, which is 50% of the rewards. I believe there is also a factor of less tokens being distributed when the price is lower and some upvotes that were at 0.01 might have gone to zero. I don't know exactly how it is calculated.