The thing is, while variance may average out over the long term, that only matters if difficulty remains the same.  If you think that I'm wrong, let me offer you a scenario. Hypothetically, let's say that based on current pool hashrate and network difficulty, we can expect one block per day. I'll offer you a choice. You can get three months of double the expected block success (two blocks per day initially), followed by three months of zero payouts. OR, you can get three months of zero payouts, followed by three months of double the expected success rate.  

The naive answer is that both will average out to the same thing - and that answer is correct so long as the pool maintains the same hashrate and block difficulty throughout the six months. In reality, even if we hold the pool hashrate constant over six months, your total payouts with the two options will be radically different due to the increasing difficulty rate. The double payouts followed by a dry spell will earn much more than a dry spell followed by double payouts, simply because you're getting paid when difficulty is lower - thus more blocks are found during a given time period.