My theory is that it came with the perfection of the power. In Cap 2 he still looked like he was getting used to having the power/ learning how to use it. Then in AoU he's mastered it completely so his hair has changed completely. Maybe it also comes with the speed itself.
This needs to happen! Deadpool will be acting all cocky and make a joke, thinking Thor won't understand him, but then he just hooks his hammer onto Deadpool and he's like, "Well shit!" Then Quicksilver has to come along and be like, "You didn't see that coming." This needs to be the next end-of-credits scene