Ah well, luck will come back
But we don't want it to get TOO high or the difficulty will go up, right?
When Slush pool has e.g. 110 % luck, it increases the global luck by some 1 % only, as its global share is roughly 10 %. That manifests itself by 1 % rise in difficulty in the next period. So it's not so bad to have good luck in our pool provided that other pools/miners have normal 100 % luck. :-)
As long as our good luck is someone else's bad luck, it won't affect difficulty.
According to the statistics, everyone's luck is independent of the other's one. Sure, when you're lucky, say, 110 % luck, and the other one is "only" 100% lucky, your share is higher. But this doesn't mean it's their bad luck. Just lower than yours. It doesn't work the way that when you have 110 %, they must have 90 %.