Bill: Well, sir, it might be because of the big Exit sign in front of it...
Head of Security: Nah, that's silly. It must be something else...

The only image I could find of this was from Fail Blog, explaining the big FAIL on it.
So to answer her question.... Maybe it's because you can tell the difference between 'They're' and 'their', or the fact that you wrote 'du u', or maybe it's because we're just jealous. Yeah, that's it. We're jealous. I've always wanted to be a snooty cheerleader like you, who can't spell for her life.
I better just hide my face in shame.
*Goes and cries in corner*