User blog comment:IamJakuhoRaikoben/Chapter 458: Morning Star, Review./@comment-26084508-20151027070737/@comment-7418318-20151027093935

Two things:

First the shot came from behind Ajeel, so he would have taken more damage, since his body partially shielded Erza. And for another, Erza both knew the attack would most likely come and saw it coming before Ajeel, so she had more time to react and throw up some kind of defense.