Here is the reason:
here are a couple of facts provided by Google (
http://www.google.com/apis/reference.html#2_3 ):
* Near-Duplicate Content Filter = If multiple search results contain identical titles and snippets, then only one of the documents is returned.
* Host Crowding = If multiple results come from the same Web host, then only the first two are returned
Now, you will find some people that will say, "well, that only applies if the filter is on." They are correct, however, the filter on is the default setting, and I challenge you to figure out how to turn it off. I tried, and gave up after 15 or 20 minutes. I highly doubt any surfer is going to go thru the effort, primarily because, why bother? They don't care.
So, the question is how do I get around this host crowding filter?
You have two options. You can put each domain on a different host, meaning you will have multiple control panels, multiple bills and billing cycles, multiple personalities to deal with . . . kind of a PITA. Or you can use someone like AaronM suggested.