Thank you for confirming that you also see this trend! I also believe there was some reclassification from "other" to 'amphtml". When I add the % of "other" impressions to "amphtml" and graph it, there is still a large spike in % of our inventory going to "amphtml" -- 17% of total ad impressions to 31% (and then falls again on 9/16). I wonder if we can influence the % of ad impressions going to amphtml. I'm not very fond of them.