It's called variance.  Other pools have it too unless you're on PPS.  It can be good or bad.  Last 50 shift luck: 71.026%, roughly 29% less than expected payout.  The only way to avoid variance (LUCK) is PPS.  Otherwise, you'll have bad times where you make less, and good times where you make more.  It evens out in the long run but most people can't seem to go more than a few days without bitching when it turns bad, then become blissfully unaware when it's the other way around and they're getting paid 30% more than they normally would because luck flipped around.