No, each of the numbers represents a completely different system (except for system 6/7 which is one system, which I assume used to be two separate systems). I think it has something to do with the technology available when the systems were built.
From what I can tell, users who regularly need to communicate on more than one system can have more than one system programmed into their radios. There are also certain talkgroups on one system that are linked to certain talkgroups on another system. I don't know of any other links between the systems (but of course, I'm not an expert).