Yup, this is another very valid criticism. I think the answer is probably to have a cutoff on the lower bound. Basically saying that for a move to be a blunder

Yup, this is another very valid criticism. I think the answer is probably to have a cutoff on the lower bound. Basically saying that for a move to be a blunder it has to leave below a certain absolute value, maybe +2 pawns, in addition to a certain amount below the best possible move.