A role is what you do, not who you are. The role should always be behavior, and not physical attributes. There are already a good number of MM entries that hardly match up statistics to their supposed role, thus when a monster's role becomes blurry it is sufficient to simply give it one of its roles, and explain in the tactics section exactly how to play the creature. I hope that it was a simple mistake, and that the intent was actually to put brute or something and that it somehow slipped through their editing team, if it is intentional then it starts to invalidate the whole reason behind implementing a role system in the first place.