Then someone probably messed up. Deleted the Center and added one afterwards, or used the Center Bone as Mother Bone and forgot to make another group for the new Center Bone. Something like that.
Can you tell me who made that model? I'd like to take a look myself.
Well, you could move the arms in Transform View (the "T" on the right in the PMXView Window), and save it, but that would probably mess up the rigging of the arms. I'm pretty sure there is a way to avoid that, but I don't know right now.
Don't worry about it, we all have to begin somewhere.