To simplify a bit, Your reward is determined by the number of shares over a time period compared to everyone else mining in the pool during that time period. So if you had 5 shares out of 100 during time period 1-5, but 5 out of 120 during 6-10, the estimate will change. Then if you had 5 shares out of 110 during 11-14 and the block was found during minute 14, then you would end up around that estimate.
So it isn't an absolute number of shares, but a relative percentage with a particular formula.
:-)
Why does my EStimated Reward constantly fluctuate up and down? If my shares are constantly increasing shouldn't me reward? It's like I gain .01 but lose .03 wtf?
