My guess is that the end of month runs look at the overall track record and browsing behavior of the visitors. Meaning, the more loyal visitors that you receive on a daily basis, the more likely that those visits will be discounted at the end of the month.
This is a scaleable way for Google to discount some of the potential misclicks, intentional clicks, and usual overpayment from advertisers toward certain specific users. For example, if a loyal visitor decides that he wants to "help" by clicking on an ad every single day for 30 days...there is nothing we can do on our end but Google will probably use some kind of algorithm to detect and remove those $. Hell we cannot even mention "ads" with any form of communication. I think that is what is happening with end of month deductions.
I was able to increase my return visits for certain sites by increasing loyalty and depth of my content. And I do notice an increased claw back at the end of the month. Last year my clawback hovers around 2~5%. This year I am hitting almost 10%'s every month. I do have a ton more mobile visitors though, I think percentile wise my mobile has now replaced more than 50%~70% of my previous desktop users, my audience's browsing habits are permanently changing, but I think those mobile clicks are more likely to be daily clawbacks rather than the monthly. Only 10% of my audience browse my sites with desktop, compared to 30% last year. So yeah...It is what it is!
In terms of observation...Business as usual.
