Actually your premise is correct.
Think about the decimal system. We call this base 10 but actually the highest individual value is 9. How do we get 'calculate the value of 99? 10x10, minus 1 to account for the zero place.
Now thin about Hex. We call this base 16 but the highest individual value is F (15 when converted to decimal). How do we get 'calculate the value of FF? 16x16, minus 1 to account for the zero place.
I know that this reply is super late and I’m probably not using the correct technical terms but hopefully it helps someone. : )