Ever see the Google Cache bring up a different URL from a different site before? Well, sometimes when a site is pretty much identical to another site with the only differentiating factor being the URL, Google might do that.
A Google Webmaster Help thread has Google's John Mueller replying to a complaint about it.
The webmaster pointed to both his site, one being at travelobuddy.com and the other being at cheapflightsbuddy.com. If you look at them, they pretty much look identical minus a few tweaks. The webmaster said, why is the Google cache showing for one of the sites showing the other site?
John Mueller didn't exactly answer the question, instead he wrote:
These sites (and some of your others) look pretty identical -- I think you'd be much better off if you just had one, awesome & strong website, rather than splitting your work across multiple potentially-mediocre ones.
Of course, one can imply from this, Google thinks the sites are the same and thus is showing them like that in the cache.
Forum discussion at Google Webmaster Help.