When copying files the vfs spefically sets the userinfo of the file. For the
file-method that means the owner and group information.

Now if you create a setgid dir of a certain group, so various people in a single
group can easily work together (which is not uncommon imho). Then this has the
nasty effect that all files copied to that dir or created in that dir don't have
the group owner set to the dirs group as expected but to the users main group...