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Just had a look at the Google cache of a couple of my sites, and they are targeted ads there.
I would conclude that on your site Google had PSA ads there when they last spidered.
I would then guess that the page cornwall viewed happened to be pages that someone previously had viewed the cache vesion of, resulting in the cache URL (ie. [18.104.22.168...] ) to be spidered and then show targeted ads when someone else views the cache later.
The pages I checked in the cache were pages that were not showing PSAs on the cache date, but showing targeted ads.
Interestingly I did notice a couple of pages where this didn't seem to work (page targeted ads instead), but by and large it does.
If you want to check it out:
1. Search for "ads by google"
2. Go to a low ranked SERP (high ones list mostly pages about the AdSense program itself - WebmasterWorld ranking well of course ;) ).
3. Click on cache links until you find a page showing AdSense, and where the AdSense ads are targeted to "Ads by Google" - shouldn't take long but there do seem to be exceptions (maybe pages where the cache has been spidered itself?)
4. Change the %20ads+by+google%20 to %20books%20, or cars or whatever. The ads will change, I promise you.
Alternitively sticky me for an example :)
I think that whenever Google hasn't spidered a page it will normally give PSAs. However if there are recongisable keywords in a query string then it will use that instead. I got the idea from a post here where someone (against AdSense TOS) put the ads on their site search engine, and it displayed targeted ads first time.
This would mean if a webmaster searches on their domain name they will see PSAs - unless someone is bidding on their domain name in AdWords. If they search on keywords that bring up their page on Google, they will see ads targeted to them - likely to be identical or very similar to those that AdSense chooses once it spiders.
The cached pages I viewed earlier that were showing PSAs are now showing targeted ads - and showing the same ads that appear on the non-cache version as well.
What you mention is also along the same lines where you can add a?keyword onto the end of a page already showing targeted AdSense ads, and it will then show targeted ads to that keyword, regardless to what is on the page. www.domain.com?dogs on a page about race cars will show dog related ads, not race car related ones.
But it still remains that if you are surfing keywords, if those keywords brought a particular AdSense-showing site up that you decided to view the cached version of, and you happened to be the second person to do so, that the chances are pretty good that those keywords you used would result in well targeted ads. I can't think of anyone who would go along and change the keywords in the URL, except for members here, LOL. But we test out things in Google that would never get done in "real life". And it still only works on pages where the cache has been previously viewed before.
For clarity, I was clicking the cache link from the SERPs. The search queries that I tried did not have any targeted ads. so the PSA were shown. This can be kinda funny. Everyone has pages that rank well for (almost) completely unrelated search queries. In these cases the cache will show AdSense ads related to the unrelated search query. I suppose this is the way they get around having to spider their own cache...
I have to admit I'm curious why. Maybe just a case of overcreative programmer syndrome at the Googleplex :) Or maybe a sign of things to come… (can't think what though)
It obviously can't have been hard to do - just plug the query string into the standard Adwords code – but it must be a temptation for content webmasters to break TOS and put AdSense on their own search engines.
I think its more general than that, as Jenstar pointed out. Any query string, it appears, will be used to target ads for a page not yet spidered by google.
I guess this makes good sense - since dynamic pages can be totally different based on the query string, Google tries to guess what the page is about based on the query string. On the other hand, one could argue that reverting to the "site default" targeting would result in better targeted ads, more often than not. For example, new pages that use "?pagenumber=123456" will always show PSAs until MediaPartnersBot comes to visit (since no one buys AdWords for "123456"), which could significantly affect AdSense earnings. The same page with a static (or static-looking) URL like "123456.html" would show the "site default" ads immediately. I guess this is another good reason to use static URLs....