We all heard the Sorting Hat say it (he? She?) has never yet been wrong.
We also watched Hermione Granger, Miss Library herself—I swear the girl sleeps there sometimes—get sorted out of the hardworking and love-of-learning houses.
And Peter Pettigrew sold out his best friends for brown-nosed protection, yet he wasn´t in Gryffindor.
And Percy Weasley. You remember Percy-didn´t want anything more than to climb the latter in the Ministry.
And Cormac McLaggen.
And yet, where did all these people end up? Yeah, Gryffindor.
So the Hat lies.
But let´s assume it doesn´t. Why are all these people there?
Well, you´ve all seen what Hermione can do. She slapped Malfoy in the face. Well, you know, she gave up school to go Horcrux hunting, endured the Cruciatus Curse, robbed a bank dressed like Belltrix Lestrange, and she slapped Malfoy in the face. You go girl! Yeah, know it.
And Percy. After all, he came back and fought the good fight with the rest of us. He watched his brother die and could´ve died himself. And he knew that.
Pettrigrew. What killed Pettigrew? His hesitation. I hear he was about to strangle Potter then that freaky hand thing killed him when he hesitated. It´s all very complicated. Voldy told him the hand would do him in if he ever showed disloyalty again, assuming Pettigrew had the brain to understand it. His second-thought moment could say a lot: he was scared to kill? He didn´t want to kill his friend´s son? He wanted to save Potter for the Dark Lord to do in? Regardless, it showed some independent thought.
So I reckon little things like that make these people what they are. Hermione can stay up all night in the library, but there´s a fighter in her too. And Percy, he´s always known what was right, deep down, and he wasn´t afraid to go after it. And Pettigrew. His is more subtle. For a split second, he dared to have an independent thought, whatever it may have been. So all of them have another side that people don´t see—the more important side, the Gryffindor side.
And Cormac? Like I said, the Hat lies.