According to the script, Curse of Despair reduces ability scores by 3, not 2 as stated. This was probably changed at the same time Bestow Curse was changed from 2 to 3 (although the in game description for that is also wrong, stating it as 2).
See line 37 of nw_s0_icursedes: "int nMod = 3; // Default mod is 3, a successful save reduces it to 1".