You earnings are decreasing, because your hubs have a different reader number each day. Also, some topics get more attention, meanwhile others barely get any views in a month. Based on this, your CPM rate is different with each hub and each day, therefore its amount can fluctuate.
When this is the case, the same amount of impressions can generate very different values in CPM, even on two following days.
I've recently made a hub on this topic, I hope I will be able to further help you with it: