Due to the nature of building binary packages and needing the users created, and packages that take the user/group combo during ./configure and then their own make install sets the user/group to what you passed in (which means the user/group needs to exist before src_install is called). We probably want to document that pkg_setup is for adding user/groups. Or at least say it's another valid place to do that.
*** This bug has been marked as a duplicate of bug 155620 ***