(from uptimebot.com which is by no means an authority.)
Google is not counting inbound links from sites with the same IP address, i.e. links between virtual sites on the same server.
Only Google policy, in shared hostings / virtual IP's is that they will reduce the burden on the server by not crawling the same address (for 2 3 domains ) at a time.
Does anyone know anything else about this? It would explain a lot but it would be very disappointing if it were true. In my case I have 3 different, very local-oriented sites that are logical to crosslink at least a little, which is how I'm doing it. One is sponsored by (a subsidiary of) the other for instance.