I'm getting the impression the AdSense crawler algo has changed with the way it matches ads to the content on the page.
I use their <!Ė google_ad_section_start Ė> and ignore tags to help identify what content on the page to match - and my code has been the same code for 3 years. A couple months ago, rather than getting ads for "left handed light-blue widgets in Anycity" I started getting just "widgets in Anycity" ads. Close, but not nearly close enough. CTR has dropped by a 40%.
I sure wish we could get additional guidance from Google, moreso than what is currently available, on precisely how to use their tags. All I've found is a lot of heresay and contradictions.
For instance, can you wrap them around just 1 word? I read somewhere that wasn't a good idea. If not just one word, 2 words? A sentence? Somewhere I read even a whole paragraph wasn't enough (despite my experience for 3 years with perfect matching on sometimes just one word).
And then how many times can you alternate between the two tags? Can they be nested or do you have to repeatedly end start end start end start? And then if you do end start end start - is that spamming?
Then it seems if you don't use the tags in just the way they have programmed in their algo (that they haven't comprehensively instructed us to use - other than "make sure there's enough content") - you can be accused/flagged for kw stuffing? I mean really? You guys (Google) can't tell us how to officially use your own tags for fear of matching the wrong keywords to a page?
Guess what... users don't click anyway if the ads don't match the content!
Then perhaps if you use the tags too much on a page, will Googlebot think, "You're paying too much attention to ads! Begone!" And then you're gone.