I somewhat eluded to this in a separate thread message, but I'm curious as to why my estimated rewards keep decreasing per round.  My miners are on 24/7 and once every two days may disconnect for up to 5 minutes due to network problems or maintenance.  However, I've noticed that my eligible share-count is decreasing, even though I constantly produce more shares in the round than the eligible count.

What is the relation between eligible and actual shares produced by a miner, and what factors affect the eligible share count?