Estimated Rewards is how many coins per day you can expect at the current difficulty, if it were to stay the same (it won't). It's based on the hash rate you put in and your power costs don't affect it.
Estimated Rewards 24h. is the average number of coins per day you can expect based on the average difficulty over the past 24 hours.
Thank you very much for that clarification. I understand the site is only an estimate. It would be nice if they could add a summary page or a "Q" near the column headings to get definition. Maybe a future revision.
Cheers!