Probability doesn't win games. Finishing, agility, anticipation, shooting, heading and positioning are skills that do. A team can be better at shooting from distance, better at floating crosses, driving through the middle or the flanks. On the other end of this, a defense also has skills that make it more difficult for shots, chances and getting in the box are more difficult. Some teams defend better in the box, some prefer are better at completely shutting opponents out of the box, some prefer to let them play on the flanks, others prefer shut them out of it. xG + xA does not truly take strategy and ability into account, and therefore is pretty useless when analyzing a game on its own.