Crayton's average is usually inflated because he calls so many fair catches, even when nobody is close to him. If he doesn't fair catch it, it's usually because he has plenty of room to run.
Rossum has called for only one fair catch out of 13 punts fielded this season. Crayton has called for eight out of 18. If Crayton had returned some of those fair catches for an average of 4 yards, it would lower his average, but it would have been better for the team.
Since 2007, Crayton has had one fair catch for every 1.3 returns. Rossum has had one fair catch for every 3.9 returns. So, if we make Rossum our punt returner and he can average anywhere close to what Crayton did (8.0 ypr) while calling roughly one-third as many fair catches, we'll come out ahead.