I don't know when supreme grade & co classification became a thing in the One Piece verse, but I doubt these weapons from the void century were considered when this was made
we need to know if weapons can be added to the classifications
surely they can cause what if someone forges a great grade blade today? do they just ignore it?
I think the amount was accumulated over a long period and can be increased or decreased depending on weapons destroyed or created
and wano was around back in void century and prior, and was famed for it's craftsmanship. So the classification most likely existed before that too. Maybe even from the first world.