uhhh....weird "talking to himself" behaviour?

okay, I'm a little embarassed to ask this, but the post about hearing voices got my thinking/worrying. My 15 year old foster son does something that I find really odd. Odd enough that I had to eavesdrop a couple of times to confirm what I was hearing! When he thinks he can't be overheard, and especially if it is when he is somewhat upset, he talks to himself. Okay, I talk to myself too - but not the way he does! It honestly sounds like he is having one side of a dialogue. His voice is in a very normal, conversational tone, and sounds like he's responding to someone else. Just the other day he was a bit shocked to find out how much he weighs (he is going to be a very tall boy) and had been talking about it all day. Sure enough, in his room later, I could hear him saying. "Yeah, but I'm nearly 210 pounds." small example, but do you see what I mean? A couple of times, I've heard him review BOTH sides of a conversation, but adding stuff on BOTH sides that wasn't there the first time. Example: We'd had a discussion that went like this My husband "You aren't going anywere until your room is clean. You've had all week to clean it, and if you want to go out you need to finish it first."him "Geez you guys are always making such a big deal out of nothing! Fine, I'll clean my stupid room". So he storms downstairs, shuts his door and cranks his music, but while he's cleaning I hear:"You aren't going anywere until your room is clean! You're nothing but a stupid little f-up and you can't do anything you're asked. You've had all week to clean it, so you better f-ing do it before I throw all your junk out in the street."...."You f-ing bastard! I don't have to do one f-ing thing you tell me to! You aren't my dad - you aren't my parents - and if you make me clean my room I'm going to break everything in there and wreck the walls." uhhh...okay. Not quite the conversation *I* heard, and I was a little worried about what he was doing in there. He comes out half an hour later, the room is as neat as a pin, and he says pleasantly "All done! Can you check it so I can go out?" A good friend of mine says that it sounds to her like he's just doing out LOUD what all of us do in our heads...thinking about what we wish we HAD said. I figure that extremely negative stuff attributed to my husband's side of the conversation must echo stuff he's heard at home, but it's still strange. What do you think...just "normal" trauma stuff, or reason for greater concern? *scratching head*