Why do I get AdSense ads on my websites that have nothing to do with their content? Only some of them have a little to do with the previous content, but I have updated it and I will keep updating regularly about every 3 days.

If adsense isn't showing adverts related to the content of the page it is one of two things.
Either google have no adverts about that subject, Or your page doesn't contain enough keywords about that subject .

Originally Posted by stOx

If adsense isn't showing adverts related to the content of the page it is one of two things.
Either google have no adverts about that subject, Or your page doesn't contain enough keywords about that subject .

But if they are showing adsense/adwords ads on your site probably the content they have in there cache is related to that terms for which adsense/adwords have highest bid for. If you have updated the content but probably it has not trggered google to cache new page due to no links.

Kinda hard to say whats wrong without knowing how your site looks.
Make sure that you dont have any html errors, and that your site is read correctly by the search engies. If the search engines cant read your pages correctly, neither can the adsense. The AdSense spider looks for keywords in your content that match with relevant ads. If you have good content, a site without any html error and no frames, it should not be any problems.

Originally Posted by zilch2k

Kinda hard to say whats wrong without knowing how your site looks.
Make sure that you dont have any html errors, and that your site is read correctly by the search engies. If the search engines cant read your pages correctly, neither can the adsense. The AdSense spider looks for keywords in your content that match with relevant ads. If you have good content, a site without any html error and no frames, it should not be any problems.

The site is www.frofi.co.uk and I am not aware of any HTML errors in it. What concerns me is that google doesn't check frequently enough for content changes. I've included a meta tag which states the site needs to be cached every 2 days but google visits no more than once a month. That only proves meta tags are really dead.

I've included a meta tag which states the site needs to be cached every 2 days but google visits no more than once a month.

Actually that meta tag means DON'T come back for at least 2 days, but as no robot actually uses that meta tag it won't matter

How long ago did you start using AdSense. It can take Google a couple of weeks before the ads become relevant.

And like Chris said those meta tags are ignored. No reason to use them at all.

OK, but is this the case with every site that's being updated often? I have even try to hide <h> text in my pages so the first thing google reads is what's relevant for my page. Instead, they seem to consider random words from <p> text more relevant. My download section doesn't use the word "download" in the <p> text and displays ads about DNA and genetics. The index page on the other hand started to display relevant ads a few hours after it was updated. I use sitemap.xml as well and my web pages seem to have no HTML errors. Why don’t they get relevant ads displayed?

One thing you can try is section targeting. It's basically leaving some comments on the page to tell AdSense where to start and stop looking for text to determine what ads are most relevant.