Thanks for the explanation Krakonos.
I'm still unclear on whether the existing shares I've contributed really will increase in reward over time, or if it's just that in theory my 'luck' will get closer to the pink area in the end.
Well, current shares would not (I think). But future shares would have higher value to compensate. Unfortunately I don't remember the correct math with extra credit, so I can't be sure.
You can see this when we hit more blocks in row, that your reward per share is significantly higher than expected with average luck. That's the pool repaying you extra credit :-).
(The Edit is because my browser decided to post in mid-sentece.)