This makes no sense to me at all. I know that hidden attributes are also important and all that. but, if the defender is defensively bad, like your CB, with 12 tackling and 12 marking, then he is.. bad defender.. He is overall bad player, with only jumping as his strength. Makes no sense that bad defender produces such good defensive performances and even wins best defender award.
This is why we need dynamic potential. If he is constantly improving, getting better and better, he can't be stuck with 12 tackling and marking. If he won Serie A defender of the year in reality SI would increase those two attributes for him to at least 15, but this way he is probably stuck to 12 for the rest of the career. He could win best defender of the World Cup and Champions League for example and still be on 12 because of the fixed potential, which makes no sense.