That's a good question.
The total rewards that are distributed on a daily basis are not always the same. It depends on the amount of STEEM in the reward pool. The amount of Steem that is added daily to the reward pool is always the same. Although, almost the same. It is about 9.5% of the virtual STEEM supply per year, but that percentage is slowly decreasing to 1% in the next 20 years.