But I find it hard to believe that anything on eHow or with Demand Media is written at an advanced level.
There's a difference between being well written and requiring an advanced reading level. If you do the reading level site: search on a major newspaper you'll probably find that most of the content has an intermediate reading level according to Google.
It would not be surprising if Google testers "disliked" what some of them could not understand, due to lack of their own vocabulary ..and if those "dislikes" became folded into Panda in part.
I think you've hit on a far larger trend than just Panda. If you study a creative writing course in the UK (and probably in the US as well) one of the first thing's you're taught is that good writing often comes down to writing on complex subject matters in a way that idiot's can understand. That's why so many of the most popular books have an "accessible" reading level(think Da Vinci Code).