Lmao. Forgot how much of a retard Filoni is. That being said you need to understand that Grievous had been injured beforehand when facing Fisto and that that Fisto's form was effective for combating Grievous's multiple blades.
As for Eeth Koth he gained the edge over Grievous with the Force, not in sabers, which means his lightsaber skill didn't have much to do with his effectiveness against Grievous.
And him performing better later on? I don't know, it's not like Grievous could have improved as the Clone Wars went on? Oh wait...