It also depends on who is looking and whether you have the "tailor ads to my audience" box checked.
I have a couple of electronic data sites that were showing irrelevant ads for stairlifts etc because the audience is people of riper years. I unticked that box, and got better revenue. Then it fell off again, and the ads lost relevance.
Golddiggerguy suggested on Acorn that using longer and explicit channel names improved performance, so I changed those and things brightened for a month, then went back to crud.
This is static material - it doesn't increase or get better, it is historic data, and if I put more up I will be duplicating other sites. But Google favours the sites that grow - even if they grow via an RSS feed adding irrelevant content by the bucketload - and reduces the quality of ads on unpromoted sites with info that is valuable to few people.
The less you muck about with a site, the more the relevance of your Adsense decreases. If you don't add pages regularly, you end up advertising crud to the bewildered and blind - at least, that's the way I see it.
Update: on the front page of a site that is detailing codes for US thermionic valve manufacturers, and historical notes about valve manufacturers, Adsense is advertising Mattresses, Spread Betting, Employment Termination advice, Removals quotes, Financial Advice, and Annuity Rates comparisons.
The ads actually get a bit better on the other pages ( about 40% relevant), but there are better focused ads in the system that are never shown - and when they were, that's when I made a little money.
