I get the feeling that AdSense is looking which pages of my site get the most traffic (ad views) and adjust the side wide ads on the subject of those pages.

I am noticing this for my dutch science news website www.wetenschap-nieuws.nl

As you can imagine this is about a broad variaty of subjects. If I don't use Section Targeting for AdSense AdSense will determine where my site is about and show relevant ads for that right?

Lately I'm receiving quite a lot of visitors who are searching for Atkinsons diet (in ditch) and so my 1 article about this diet get's a lot of pageviews.
I'm now seeing ads about weight loss and diets on other postings. So it seems that AdSense thinks my site is about weight loss and dieting, unless I tell them otherwise with Section Targeting for AdSense

So from this I came to the conclusion that subjects of higher visited pages are more important to AdSense in determining which ads to show than the lower ones..

are you seeing this as well?

ubjects of higher visited pages are more important to AdSense in determining which ads to show than the lower ones.

I agree with the logic of this statement.

And, lacking a ton of empirical evidence, my gut tells me it is so.

I have asked Google this question. Let's see what they have to say about this

Yes, that sound possible, maybe even plausable.