I dont think that the AI would give Giggs and Beckham the same instruction, but i'm sure that it will give them standard instructions from the TC (winger with attack duty to giggs, and perhaps wide midfielder with support duty to Beckham).
We, human managers, work from those standard instruction but tweak them according the player attributes.
This could be the main reason why players play better for human teams. Despite, i still believe that we must not generalize because like i said in my post about Nicolas Gaitan, there could be logical reasons why the player plays worst in the AI team.
Perhaps this could be the next step for the AI: starting from the standard TC instructions, have the ability to tweak them to take the best from the players. (this is valid for the team and player instructions)