I noticed that "estimated rewards" slowly drains over time if you aren't mining
, or haven't found a share. What is the meaning of this?!
Simple. It's a proportional payment system. If you aren't mining (or sending invalid shares), then the proportion of shares you've found as a percentage of the total shares processed gets lower.
The only way to maintain the Estimated Payment is to keep providing the estimated number of shares per minute. That's why it's an estimate and not a guaranteed. If you were on a PPS (Pay Per Share) system, then it wouldn't be an estimate, as you would receive payment for every valid share submitted.