Why am I not receiving STRAT as often as I would expect?

Staking is random so the expected time to successfully stake is an average estimate. Over a long enough time frame the average time to stake should be reasonably accurate. Over shorter time frames it could be quite inaccurate. At times you could be waiting three or four times your expected time to stake and at others you could be successfully staking three or four times more STRAT than you expect.