1.5s are a special category with a unique name (“fair”). The grading standards were designed with really old tobacco cards in mind. I actually collect some of them from the 1880s
These are cards where PSA 6 is genuinely the highest grade you will find. I guess the 1.5 is for cards that are really bad but not absolutely trash. Like passing a school test with 51%. Technically a pass but maybe nothing to be proud of
Because the tobacco cards live in the lower half of the grading scale I guess PSA felt the need to add something between “absolute garbage” and “not terrible” particularly for cards like these