Wait, since when does Google care about the extension of a domain?
I am not speaking from anything but pure speculation here. So, my assumptions/guesses may be dead wrong. The caveats aside, I can see why the extension would make a difference.
I am thinking that there is a hierarchy of more popular extensions (or perhaps, more trusted extensions). Everything else being equal, I think it is safe/non-controversial, that if someone is searching on a search engine, they are more likely to go to a .com extension than, say, a .info. This hierarchy is probably quantifiable (but, not by me). I think, for example, seeing a .ru extension may not be trusted unless you are from that country.
My point is that if you are not already searching for a specific site, if you put in search terms, you are likely to get .com extensions in results before other extensions (except, perhaps, for very specific sites that already are well known- like wikipedia.org...but, back to my point, you don't search for that site based on terms, you search on the URL portion of the name...like "wiki"). If I am not way off base here, I think that you end up with a "chicken or the egg" type of problem. If you have a .com extension, I think people would be more inclined to go there in the first place. More eyeballs, more members, more posts, more traffic, and the result is somewhat the result of circular logic. Because you have a .com site, more people may go there...and the result is that google may tend to show .com sites more relevant because of the effect of more people going to sites with .com extensions.
I think you are right, though, that in the abstract IF more people are going to a site with another extension, then google will show that result higher than a .com site where people are not going. But, I tend to think that in reality this does not happen in most cases.
Or, I could be wrong.
