My theory about alt text showing as correlated to higher ranking is that it's a result of people optimizing their pages. SEOs have been saying to add alt text for a while, and it's easy to do compared to changing URLs or a domain name. It's hard to prove whether it's a cause or an effect, but my gut feeling is it's an effect.
Lots of data there, I wish I were smart or educated enough to know what it means. Totally unscientific of course, but I've not found Bing to be as good for me. Then again, maybe I'm just more familiar with Google or they have subtly influenced what I think I need to what they already provide.