My guess is that the system is set up so "groups" can be created dynamically in the same way posts are.  This likely answers all your questions.  It's the date the group was created and the group was created by the system user "Community Manager".  This also explains why the index loads dynamically instead of assuming a finite number of groups.

I completely agree though, this is not useful information.

Update: You might also notice that this groups index you linked shouldn't technically be accessible.  The preferred view, as navigable from the menu, is the "community" view: 

Thanks for the quick reply.  Can you provide any information on the team or individuals responsible for the community?  Do they have other responsibilities?

Any thoughts on why simple, obvious things like the quality of the main banner image have not been fixed?  Is there a set development cycle?

Sorry if I come across as rude, I'm just curious and want to help the site improve.