Sales numbers don't back this up at all. If a sealed game is decidedly in great condition and looks like it will grade A+, 9.6, whatever, people will pay a lot for it. But, if that same game is already graded and stamped with an A+ 9.6, it will sell for markedly more. This is why people buy sealed, grade, and flip. It always helps from a sales standpoint to have a third party give their input on condition, especially if that third party is recognized.
Now, if you want to talk legitimacy of grading companies, etc., that's a whole different story.