I don't know that he's regressed but I don't think he's improved much since his draft year. The Kitchener Rangers have arguably less talent these past two years than they did in 2010-11 and that could certainly explain some of the regression.
Don't rule out the possibility that Murphy is getting bored and/or working on other aspects of his game. You spend so many years dominating at a certain level and you might starting looking to do some things differently just for the sake of it. And no doubt Murphy wants to develop his game so he can play at the next level. I don't know that this is necessarily the case but I wouldn't rule it out.