Off Hand Only Why?

[Motenten]

You actually have 3 rates and 2 composition factors.

The proc rate of the weapon. Always 30% for all weapons at 100 TP.

The number of base attacks possible per round.
2-handed weapons: always 1
H2H: always 2
1-handed weapons: usually 2 because of DW, but can be 1

So in 100 attack rounds, the number of ODD procs would be:
2-handed weapon: 30
H2H: 15
1-handed weapon: 15 or 30, depending DW

Since 1-handed weapons are -usually- DW'd, you can generally expect that, for example, Kannagi will get the same proc rate as Verethragna, both of which are half the proc rate of Ukon.

Given the relative strengths of the weapons, I would be inclined to suggest a 50% increase in ODD proc rates for 1-handed and H2H weapons (ie: 45% proc rate at 100 TP instead of 30%, up to 75% proc rate at 300 TP instead of 50%). That would still leave them weaker than 2-handed weapons in terms of ODD procs, but helps make up for the fact that they only get half as many chances per round. And while single-wielding 1-handed weapons then gets them a significant increases in relative ODD rates, they'd also be giving up the rather significant advantages of Dual Wield, so the gain (if any) is not nearly as large as it might at first seem.