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It's indexed to the value of STEEM. The value can fluctuate. It is in the FAQs: https://steemit.com/faq.html#Why_do_the_earnings_for_my_post_go_up_or_down

As other posts in the system get better upvotes, and as the payouts fluctuate, there is going to be a difference between the day 1 and day 7 estimate.

The value next to a post's voting line are just estimates. There has been a downward trend for about a week for everyone as #hf19 has taken effect and people vote and post differently.