What makes this an easy play for a referee to botch is that McGahee lands awkwardly. Usually a runner's knee will hit the ground first and then the rest of his body will come down, and it's often in that split second after his knee hits but before the rest of his body hits that he'll reach the ball forward, and the extra reach is correctly subtracted from the spot. But look closely at the play. In this case, Willis did a great job of keeping his knee off the ground until the rest of his body came down, and the extra lunge forward came before he hit the ground, and he did get the first. At least thats what I see.
I can't get on the refs here though, its a very difficult call and easy to get wrong. In most cases that spot would've been correct, but Willis put forth a good effort to keep his knee up which I believe went unnoticed.