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I run two sets of analytics on each page - both Google Analytics and Clicky. Both are close enough to each other to be believable - both are in the 600,000 range for December to date for one site.
Yet, Google Adsense page impressions is approximately 100,000 less.
100K impressions gone missing could be some significant $$ gone missing.
I realize that ad blockers could explain some of it, but I doubt it would explain 1/6 total. Those of you running GA, do you see similar discrepancies?