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.

The upshot of this is.. over 5 days I've got 21% of what I expected :/
I have some other rigs pointed at btcguild and I'm getting the expected returns for my hashrates.