Using this Skill

The final attack of a melee combo is the same skill..?

Control Marionette allows you to take control of your marionette and play from its point of view.

I'm not sure if its applies to every weapon type but I've observed the fifth attack of a knuckle combo is usually higher than the max damage shown in the character window whereas the other four are of the range max damage that is a hidden stat. That might also explain why you can knock back with a final combo attack but idk. All I know is that it is abnormally high for an attack that's not a skill or a critical hit.

It doesn't have to be a final combo to knock back/down. Knock back is done by knockdown gauge. As for the damage, I don't think so.. damage should always be the same formula, if not, it's still the same skill. There's no alternate version. The calculations are just done differently (if so).

Then perhaps there is a graphical error that shows the wrong numbers?

That's more unlikely than a 2nd damage formula. Lol..