I studied it and came up with a theory watching search results in Google Analytics and Webmaster Tools. I think that there is actually an order starting with the home page, then in order of importance, as Google sees it, from most important to least. I see some reordering in the SERPs that seem to coincide with changes in search traffic. I also think this theory may fall apart after a few SERP pages mostly due to lack of traffic to these pages.
Now this is just a theory and I could be mad as a hatter on this. I am still watching and learning on just this very question because it puzzled me too. So take what I am saying with a grain of salt. However, it does seem to make sense since the SERPs are always based upon order of importance with a bit of mixing it up to include resources from other sites. Having said that, when you use site: you are in effect eliminating Google's ability to mix things up leaving only the order of importance. The one exception seems to be that the sites home page is always listed first.