A common question here is why your Adsense eCPM drops down. Here I will give a list of the most common reasons but feel free to jump in and add any more I have missed. Your traffic source has changed and people are clicking less ads. Your traffic source has changed and clicks are not converting for advertisers, and therefore Google reduces how much it pays you and sells cheaper clicks to advertisers on your site. Google has identified content on come pages on your site as being unsuitable to show ads, and a lot of public service ads are showing on your site. A particular page on your site might have been driving a higher eCPM, and traffic to that page has reduced, the quality of the traffic has changed or the ads on that page has changed in some way. An advertiser was paying a higher than normal rate to specifically advertise on your site (either the entire site or certain pages), that advertiser has now ended their campaign. (when you see those single Adsense text ads that take up a whole ad unit they are often advertisers that are specifically bidding to advertise on your site - they pay more). Due to your traffic quality you send to advertisers being poor for a prolonged period, Google has reduced what it pays you and charges advertisers less. A traffic source that was raising your overall eCPM has now stopped or being reduced. For example, you may have been getting a lot of traffic to one page for a high paying keyword, and now that traffic has dried up. A change in the market means that fewer people are now advertising on keywords related to your site. Your pet cat, which secretly hates you, broke into the Google headquarters and lowered your eCPM. Advertisers bidding on keywords showing on your site have stopped advertising which means the CPC drops. Recent changes you have made to your site have effected what ads are showing (such as changing content or what is in the meta title tag), and the ads showing now are paying less or getting a lower CTR. Google has made changes to its algorithm which decides what ads show, and it is now showing ads which pay less or get a lower CTR. The overall quality score of the traffic you send to Google has been downrated as it has not been converting for advertisers, so Google lowers the cost to advertise on your site and/or reduces its revshare with you (I'm not 100% sure this is true but I've heard rumors this happens) Did I miss any? Remember that eCPM will still jump around a lot for all the reasons above, so it is best not to obsess about it, as in most cases it is out of your control. Obsessing on producing a great website and doing effective marketing is much more important.
What this joke meant? "Your pet cat, which secretly hates you, broke into the Google headquarters and lowered your eCPM."?
I think the OP means don't get obsessed by ecpm because it is beyond anybody's control. Just stop thinking about it and keep building good content.
I think you missed the case where a change in ad placement (accidental or not) has lowered the CTR. eCPM is also directly related to CTR.
I completely agree, I don't know why people get so hung up in their stats. Much better to stay focused on your work then worry about analyzing why your eCPM dropped. Just another work avoidance tactic as far as I'm concerned.