Have you ever successfully scanned a QR Code that had been beaten up by the wind, flecked with dust, or "visited" by pigeons? These codes can still function due to something called error correction. There are different levels of error correction that can be used in QR Codes.
Bitly uses Level M.
Here's a helpful bit of information from QRStuff.com. There's an indicator at the bottom left of every QR Code which denotes its level of error correction.