I don't buy the advertiser explanation.
If this was the case, there would inevitably be times when you have good traffic AND good earnings ecpm/cpc.
Traffic up, earnings down, with no exceptions that I have seen in the last 5 years I've been running adsense. When I write/post some excellent content that earns links and traffic it has always been followed with a reduction in earnings. Then after a week or two when the traffic dies down, the earnings always go back up.
This is clearly an algo. They appear to baseline sites to get a 'normal' reading on impressions. Once your traffic shows a significant increase the algo ALWAYS kicks in to knock the cpc way down. If the traffic stays high for a month or more, then your baseline is moved up. If I then get a day with only a little bit of traffic they give me insanely high cpc's. I've been watching this for years.