As far as I know the maximum amount of co-op ads displayed per page is 5. So far I've been displaying 4, but today I switched it to 5, recalculated the weight and to my surprise the weight remained the same. I'm a curious being so immediately I tried to display 3 ads per page. The weight hasn't changed at all. So I switched to 2 ads per page - this time the weight was lowered by something around 10%. The obvious choice for me was to stay with 3 ads per page, but my question is why do I get no additional credit for displaying 4 or 5 ads ?
And that footer of mine is the only place where I have the ads code, so if it was not attached to every page then I would not be able to validate at all.
It seems no one is interested in such issue so just for your information - I've spent some time checking things and finally flushing the ad_network_ads.txt solved the problem ;] Now the weight I had previously while displaying 3 ads I have with 5 ads per page. And with switching to 4,3,2.. ads the weight changes properly. It seems that flushing is a cure for majority of riddles & mysteries