It's my understanding that the moveset grades are relative for that specific Pokemon, so you can't really compare them like that.
Every Pokemon has an A rank moveset, even if that moveset is kind of crappy. For example, Kingler's 'A' rank moveset is Metal Claw/X-Scissor. If you added that as a 4th option for a better Pokemon (like Flareon) it would certainly be an F. Alternatively, if you gave Kingler Water Gun/Hydro Pump then it Metal Claw/X-Scissor would surely drop to F
Another example is Ember/Fire Blast which is a 'C' rank for Charizard but 'A' rank for Flareon. This just means that for Charizard, better options are available.
So the ranks are only really useful when comparing moves on the same Pokemon, and to give you an idea of how much better/worse one particular combo is. Going from A to B isn't a big drop, but A to C or D will be.
HTH
