This is just a random thought and experience: I have 2 large ad squares on top of my page, and a horizontal text link ad in the middle. You'd think the CTR of the 2 large squares on top would get the higher CTR, but Noo...my single, small, 728x15 link unit in the middle has a 3X higher CTR than the two large units on top. Kind of strange, huh? But I could care less, whatever or just as long as revenue is coming in.
Usually the ad format that blends well into the content gets more attention. Probably your reader have started to browse through and read what you have until reaching the middle before clicking on the ad.
Ad links has performed well on my site too. But, this takes some experimenting to see which Ad Type works well for you.
I think the Ads should blend with contents. I also ready somewhere if the info in your site is not 100% complete/satisfactory people will start look elsewhere (go back to search again or click on an Ad).
Sometimes, link units performs much better than ad units. In my sites, I have only used ad units, as I tried link units and they did not worked good. On the other hand, there are few pages where link units perform better than the ad units.
I prefer large rectangle ad format blended suitably with contents, which perform better than any other formats. Of course, it varies depending on the site layout and niche...
Different niches respond differently to various ad placements. Positioning a large rectangle in the center of the page above the fold will usually get lots of clicks. TomG.