It IS plausible the way they do things leans towards the most enthusiastic usually getting to cover a game, and as such is inclined to rate a game highly. Imagine if you mainly only got the people who call whatever GotY to write a review on a forum like this, you would definitely be seeing plenty of 10s and 9s. Which is part of why Famitsu is more blatantly suspect, it's one thing to get the wild fan to rush through a game and score it high before that high wears off (if it does), it's another for 4 people to have similar scores and just write a sentence or two about the game.This.
And this.
Why IGN? Because they have some strange scores and practices. They need that ad revenue afterall.