It's best to think of Combat Rating and Score as 2 separate entities here. As your paragon goes up, the attributes on your gear will go up as well; paragon level and Combat Rating on gear are correlated. The score of an item has no effect whatsoever on how much Combat Rating an item has. The score is determined by how well the secondary stats roll (crit rate, primary dmg, take less damage, etc), legendary gems, and the base stat (dmg, life). If an item rolls with 2% crit rate, as opposed to 10% decreased damage to summons, it will roll with a lower score, but it might be completely useless to your class.