The difference is one word.
You may not realise it but McClain is no where near Fua in the run game and Fua is no where near MCClain in the passing game. Neither excel in their strongest areas, they should improve, but to have them one degree from average is just wrong.
Using your rather limited way of 'grading', McClain would be very poor against the run, below average pass rush and Fua would be average in the run game and a very poor pass rush.
When it's an extremely short evaluation, one word carries a lot of meaning, as you well know.
McClain was very poor against the run during the pre-season, but imporved some during the regular season, which is a fact that is common knowledge to most Panther fans, and obvious to see on the field. Hence your grade is incorrect.