Which explains why 2 sites with roughly the same traffic and CTR can result in 1 site making $10/day while the other site makes $200/day.
The source of that traffic is also a factor of the quality of the traffic, yet another consideration impacting EPC.
Newspapers and magazines also didn't have millions of people using Firefox running the AdBlock add-on automatically zapping AdSense ads either.
Ads missing in the first place due to technological interference is far worse than "ad blindness" where you still have a chance someone might see the ads.
I think part of the decline in ads can easily be attributed to the increase in Firefox usage and those add-ons.