Somehow this troll succeeded in injuring a student, without alarm from the wards until the point of her death.
So someone tricked those wards, wards that were apparently working when Draco Malfoy was attacked. (Someone tricked them before, but that was by killing him so slowly the wards didn’t notice, not by disabling them so no one noticed a student was in mortal peril.)
I wouldn’t infer from this that the culprit was necessarily different, since after all in the case of the attack on Draco, the intent was to frame Hermione, and thus the methods used should not have involved anything Hermione herself was not capable of.
So someone tricked those wards, wards that were apparently working when Draco Malfoy was attacked. (Someone tricked them before, but that was by killing him so slowly the wards didn’t notice, not by disabling them so no one noticed a student was in mortal peril.)
Of course, the attacker could also have chosen to kill Draco slowly in order for the ward circumventions to go unnoticed...
I wouldn’t infer from this that the culprit was necessarily different, since after all in the case of the attack on Draco, the intent was to frame Hermione, and thus the methods used should not have involved anything Hermione herself was not capable of.