I am curious how many keywords you have been tracking and for how many months you have been monitoring them.
I monitor keywords for products I write about, 2 posts per day on average, since I started to notice the problem after the Panda 1.0 (I had some pauses in monitoring because I got bored with that, but the things remained the same all the time since February). On Sept 28th things improved regarding this, but after 2-3 days the problem occurred again.
So, I was tracking almost all of my keywords, trying to figure out why MFA re-writers rank better, but couldn't figure out why. They reason could be that they have "better" ads than I do, because the content is almost the same as mine, with minor changes. I always get natural links from respectable sources in my niche, and newer saw any of these MFAs mentioned on any site. Used backlink checker tools to determine if tey have quality inbound links - they have not.
I think that there could be some logic in favoring Adsense by Google, because they claim their ads are "relevant and engaging", so why would they de-rank someone if uses relevant and engaging text with links to relevant sites. On the other side, they can't guarantee that ads by other sources are relevant and engaging, so their use could be a problem. After all, in Analytics Google lists Adsense data under "Content" section, so they probably consider their ads to be content.
This is all speculation of course, but I can't see any other reason why a MFA website which only re-writes others' content outranks the original source.