There are two possible reasons for yield fluctuations:

Hashrate Fluctuation: Hashrate output is unsteady and naturally fluctuates during mining. Factors such as energy, network, miner status and more can also affect hashrate, resulting in a change in earnings. 

Difficulty Adjustment: Mining difficulty is adjusted after a period of time, with most adjustments being an increase. When an increase takes place, miners will experience a reduced mining yield even when the original hashrate remains basically the same.