There's got to be some kind of loophole. I'd like to know what they're counting, exactly.
A big reason why it sold so much (aside from the series being very popular) is that it released at the very end of the last gen and then was re released early in the next gen so a lot of people just bought it again on the new consoles along with people new to the game.
We've also got the fact that a new GTA hasn't been released since. All the other GTA's had there legs cut off by a newer release in a relatively short period of time.
The game is also very regularly on sale which should help it's sales a lot.