Ya you're right about the spoilers. I edited my post and added those numbers in as an afterthought. But the bottom line is that the 320/321 are much quicker to descend than the 319.
As for the exact aerodynamic reason why, I'll leave that to the experts. I don't have a clue. But I guess we can rule out both weight and spoiler deflection as the figures I posted are for the same weight and it looks like spoiler deflection is the same for all aircraft with AP on.
The other thing that came to mind was the fuselage drag, but looking at the numbers the 320 descends quicker than the 321.
Last edited by Airmann; 5th November 2012 at 17:16.