I just receive an official answer from Support . They said that the reason behind this difference in quality calculation is that the defensive skills are not 1 as shown there but in fact they are negative , very much negative , and that's why the general quality is dragged down . Now my next question is how the game determine a player's performance that has negative skills? I bet the negative skills are calculated as 1% when it comes to player's performance . So when it comes to establish the general quality the engine takes into the equation the negative skills but when it comes to performance it doesn't !