well actually you might not have been so wrong there, i do correct myself again.
i got curious about this definition when i got home, so i checked out the NBA's official defintion.
NBA's efficiency rating (PTS + REB + AST + STL + BLK - FG missed - FT missed - TO)
So i double checked a game of BB, did the sum above and got a different number to the +/- that was listed against the player
So i can only assume that +/- is the team score differential for that player when he is on the court, which is a different stat to what is in the NBA.