I don't know... I think that these debts also have something to do with perception,and gratitude...
ie, though Snape hated it, he acknowledges that James saved his life, and Wormtail certainly knew that was it not for Harry's pity, then he woudl be dead too, and if grateful for that... whereas Voldie feels none of that... Peters hand was just a means to an end, he's not really grateful for it, but woudl rather feel that he deserved it...
Okay, that really doesn't describe what I'm thinking very well... but I can't think of a better way to put it just yet???
