Each service and or pulbication has the opportunity to call whatever game they want the GOTY. It's up to the consumer to decide if it matters at all.
I know some people that will only recognize places like gamespy or g4tv etc. But all one needs to do is look at any number of non mainstream sites and they all give out their GOTY award to somebody too.
And the funnier thing to me is that some smaller non AAA titles will gladly accept GOTY from whobangedmygame.org.can.momma.com and print it on their boxes as the title is a GOTY.
Well, those smaller non-AAA titles sometimes are more deserving of a GOTY than true AAA titles - but the heavy hitters like TES, Modern Warfare, Halo, Zelda, etc tend to win purely on name alone.. So if I was the little guy, I'd print GOTY on my game box too if someone gave it to me.