Hi zarney,
On first look, I would say you need to change your criteria to NOT use the "Assignments.CreateGroup.Title" attribute. This is probably why you are getting some listed that should not belong and vice versa, since it's associated with the creator of the assignment and not the assigned group itself.
Instead have it look at "Assignments.Group.Title", and possibly "Assignments.User.Group.Title" (in case Incidents might get assigned to an individual and not a group). Your naming may vary but you get the idea, use the assignment's Group and User/Group instead of using the CreateGroup.
Good luck.
-Brian