UK Adsense

Why on earth is it that it seems UK users are always served the very worst of all ads when sites are using Google Adsense. It seems to me as a UK browser theyre irrelevant on a lot of sites and are always things that no one would really want to click on. Im sure this has nothing to do with Adsense targetting at all, it really does seem like we get the crap end of all possible image ads. The only ones that are REALLY relevant usually for me are the text ads.

If I switched to a US proxy where ads will show theyre normally good things that I would be tempted to click on especially as a member of a forum. I have also noticed the ads served to the US tend to vary more often, the UK are served a set per site or so it seems and they rarely change.