AthlonInside - 3:17 pm on Aug 15, 2004 (gmt 0)

DaveAtIFG,

I notice similar case as yours in one of the sites I monitor.

IMHO, the reason for this due to the implementation of Local Rank. In local rank, Google doesn't want a few sites from the same owner appear at the same time in the SERPs for a search. That's the possible reason why only www or your subdomain website appear. This also apply to webmaster that have a few websites (different domain) that rank well for the same keyword, now I notice they are filter to one site left in the SERPs.

For some people who read this, they will definitely ask how Google knows which sites are from the same owner. There are not very smart well on this but they are using 2 not-so-good but good-enough methods.
1. WHOIS
2. C Class IP Address