And yet the explanation for the method of the attempted murder of Draco was that the slow cooling of his blood would cause his vitals to drop too slowly to trigger the wards until he died. Which explicitly relies on the common knowledge that Hogwarts DOES have wards that track the vitals of its students and that those wards are keyed to track sudden changes, and the removal of significant portions of the body would certainly constitute a "sudden change" in vitals.

So in the attempted murder of Draco, the wards were circumvented; in the troll attack, they were actively compromised.