I think you're reading the tooltip backwards. What it's saying is that the damage multiplier changes based on whether the weapon's listed Armor Threshold is Above, At, or Below the target's Armor value. So if we take the Herbicide as an example, with its Armor Threshold of 2 and its listed Above, At, and Below multipliers of 0.4x, 0.9x, and 2.2x, what the tooltip is saying is:
If the weapon's Armor Threshold is Above the target's Armor, the damage is multiplied by 0.4
If the weapon's Armor Threshold is At (or, more coherently, Equal to) the target's Armor, the damage is multiplied by 0.9
If the weapon's Armor Threshold is Below the target's Armor, the damage is multiplied by 2.2
That's the way it's supposed to be read, and accurately reflects the way energy weapons work in practice; the Above, At, and Below statements are referring to the Armor Threshold of the weapon relative to the target's Armor, not the target's Armor relative to the weapon's Armor Threshold. So the lower the weapon's Armor Threshold and the higher the target's Armor, the more damage the energy weapon does.
It's also pretty readily apparent but still notable that some energy weapons will never do their listed damage. The Herbicide has a listed damage range of 10-19, but no matter the Armor of the target it will deal either 40%, 90%, or 220% of that. The base damage range will never, ever be what it deals, no matter what the armor of the target is.
Complicated things, these death rays.