I honestly feel as if Microsoft is paying them to give it that spot. Their is litterally no other semi-logical reason or explanation on how they think Halo 4 is the third best game, out of every single game on Xbox 360. It literally doesn't make any sense.
I think its a combination of mutual back rubbing and being completely clueless.
I'll give you an example. IGN reviewed a PC game called Rise of The Triad which is a modern remake of an old school FPS. The game is extremely fast and quite difficult. In the video review something like the following is said:
IGN numpty 1 "So you like old school fps right?"
IGN numpty 2 "Yeah, I really like old school fps"
The guy (numpty 2) who is playing basically plays the game exactly like CoD, running straight forward with little strafing and aiming down sights all the time (ADS makes no difference to how guns perform in ROTT and is purely visual). He gets utterly destroyed on normal difficulty as he has no idea how to play the game even though he is apparently a fan of old school fps. Its hilarious.
He then gives the game a really bad rating for reasons that don't even make sense. For example he heavily criticises graphics which while not amazing are reasonable and certainly worthy of 7/10 overall. He also criticises the AI which is generally solid but nothing special even though again he can't even get through the first level of the game as he keeps dying so its certainly good enough to get the job done.