You've come across an interesting point, here's what my MEL says, it includes A320s/321s/319s (new and old)
All A320 & A321:
With Speedbrakes available: No ALT restrictions
Without Speedbrakes: 31,500ft.
319s
With Speedbrakes available: ALT restricted to 37,000ft.
Without Speedbrakes: 31,500ft.
Probably has to do with regulations.
With speedbrakes available, the A320s and A321s can achieve a high enough rate of descent that in the event of the loss of the remaining PACK the aircraft can reach a safe altitude quickly enough from max ceiling (39,800) not to necessitate a limitation on the alt. The A319, due to it being lighter, cannot achieve the required rate of descent and needs to be limited to 37,000ft, even with spoilers available.
Without Speedbrakes all aircraft are limited to 31,500ft.
This is my guess.
Last edited by Airmann; 5th November 2012 at 14:28.