To make it more clear:

Expected behavior: if the pool is unlucky, as my shelved shares are deeper and deeper in the queue (i.e. in the past), during the lucky phase at the start of each round, I will see 'estimated' graph at the start of each round with a lower threshold above the paid+unpaid graph, and decaying faster and faster, until I fall too deep in the queue and I reach the moment when it becomes flat.

Observed behavior: until June 26 I have seen expected behavior as described above, but it was far from becoming "flat" at that time (edit: unfortunately now we can hardly see it on the graph, it's too much in the past). Since then, the "estimated" graph is flat. It was all of a sudden, and it coincides with a short period (tens of seconds) when I submitted many shares, after being idle for long, and the staying idle for long. The hashrate graphs are correct (I currently no longer mine with that address, the spikes are just testing), but on the rewards graph, I can't understand the "break" of June 26.

Edit: another thing, the shelved shares date back from some (2-3) weeks ago, not months.