That's the point the others are making too ... all of that description is just flavor text; there's no rule mechanics attached to it to say "5 points of damage = leg severed, can no longer walk" or whatever (pick up Bastion Press' "Torn Asunder: Critical Hits" when it comes out if you want rules for that sort of thing, BTW).
I do the same thing in my game, as DM, depending on the creature's HP and damage dealt ... "You strike the creature a glancing blow, opening a gash in one arm" (hit 4th level fighter for 5 hp damage). It isn't until the creature's last HP ebbs away, though, that the climactic blow is struck: "You decapitate it ... you sunder the construct's leg, and it falls ... your rapier pierces the creature through the heart, and it drops dread ..." etc. But again -- that's all flavor text.