Actually Ball was +8, Ross was -13.
The +/- Stat indicates how many points a team gained or lost relative to the opponent when a player was on the floor. For Example: If Player A comes in the game and his/her team is up by 10, but they are tied when he /she exits the game, his +/- for that stretch will be -10.
So, while Ross may have appeard to play good defense (and maybe he did on his man) while he was on the floor, the Huskies lost 13 points relative to what the score was when he entered the game)