Amount of conductors in a wireway...

[GoldDigger]

I follow your theory, but here's where I'm not ok....if there are 2 conductors, they're both current-carrying....whereas with 3 conductors, it could one OR 2 current-carrying conductors, given "most" branch circuits.

You are persisting in confusing the fill requirement, which is a purely mechanical issue and involves the overall size of the conductors, including insulation with the ampacity derating, which refers to the number of conductors, each assumed to be carrying its full allowed amperage and contributing to the ambient temperature seen by the other wires.

The mechanical fill percentage does not care at all about whether a wire is current carrying or not. It occupies the same space either way and you have to pull it whether it carries current or not.
What steve66 stated is that one conductor, with sufficient clearance, can take up 53% of the space in a conduit.

Without looking at pulling or other NEC restrictions, the maximum area that a single wire can occupy is 100% of the cross-section.
If you put in two wires of equal size, the diameter of each can only be 1/2 of the conduit diameter. That means that the total area for two is 50% of the cross section.
But if you put in three wires of equal size, each wire will have a diameter equal to 1/2.15 of the conduit diameter, and the total fill will be 64.8%. As you increase the number of wires, the total area that they occupy continues to grow up, but never getting above somewhere around 90%.

(If you are allowed to use wires of different gauges to fill in the space, you can theoretically get back as close as you want to 100%.)

If you take the greatest possible fill, and then make reasonable allowances for free space for pulling, you get the numbers that are found in the NEC table.

You can see some helpful illustrations on circle packing here.