A320 Spoilers
Something slightly different I heard.
The reason for the difference of speed brake extension A320/21 is during testing it was found that during an emergency descent with max speedbrake on the A320 that the resulting pitch up moment exceeded the autopilot authority to correct it resulting in it going into a protection area (guessing alpha prot) and the autopilot disconnected. Thus the A320 has less speedbrake throw than the A321.
Could probably go dig up some paperwork to confirm it if anyone thinks its a tall tale