I wonder what West was thinking during this auction. If his plan was to bid 5♣ over 4♣, why didn't he bid 5♣ in the first place?
ATB: In this case it's Attribute the Blame (for us) and Credit (for them). You cannot blame South for not opening. South could (should?) have bid 3♥, but if West was planning to go to 5♣ anyway, this has no impact on the score. For failing to double 5♣, North receives all of the blame that you're not assigning as credit to West, who knew to bid 5♣ instead of defending your vulnerable game.