Dogecoin has a varying block reward. Coinwarz averages it at 500,000 because right now the reward can be up to 1,000,000 coin per block. If you the pool you mine at has a run of sub par blocks meaning less than 500k per block average then the calculation will be off. The difficulty fluctuation also come into play as well but it's mainly the random block reward.  You could have block hit for 900k+ then 4 more after that below 50k.  You just never really know.