Here are the admob CTR/eCPM and / impressions graphs for about 9 months of an app I developed way back. I left the scene for two years and now came back and researching the advertising frontier.
so I’m now looking at what happened during these 2 yrs. it seems number of impressions stayed the same but $ per day went down from 3 to 0.3 
I can’t explain the peak in the CTR.
good luck to all,