This was my initial thought as well. However the spartan reports definitely call the smallest squares sectors. So there are two problems:
1. If "Galaxy X,Y" names a square within the galaxy, why does "Sector X,Y" name a specific sector, as can be seen in SPARTAN reports and the "unexplored sector" description, etc. It would seem both naming conventions should work the same way, i.e. "Thing X,Y" should in both cases name a square within the "thing", or a "thing" itself, not the one in one case and the other in the other case.
2. If the large view is a single galaxy, as it appears, and the smallest squares are sectors, as they are called, WTH do we call the middle size squares?