What percentage of players perform better or the same at a new club than they did at Melbourne?
You could say the same about players from any strong club. They always look better surrounded by other good players.
So in a team full of players who are 7-out-of-10 for quality, I reckon a few of them will look like they are 9/10.
Then when these players go to a club where most of the top 17 are only average (4-6 out of 10) there is more pressure on them, and less opportunity, and it becomes clear they're not as good as they looked in the team full of quality.
You see it all the time at clubs that lose a few top 17 players to injury or just cap pressure. The 'stars' who remain (e.g. DCE at Manly, or anyone currently at the Roosters) look like they've lost ability. The fact is they've been shown to be less awesome than they seemed.