Oh, I won't disagree with you at all because you are 100% correct!
The diameter of the bar is what drives the initial soft "setting" on bars. Like you stated, a Whiteline 27mm is 27mm in diameter. Here is where swaybar math gets fuzzy...
The increase in bar stiffness comes from the location of the mounting holes on the bar...closer to the bar, the
stiffer it is. Now, how much stiffer are these holes? That is where the construction of the bar comes into play as well as the location of the mounting holes. This is explained in a percentage increase.
By using the known effective bar rate of 27mm (let's make this easy and say its 500 lbs of effective rate), Whiteline can say that the next hole mounting location is 50% stiffer. Well, that translates into a 750 lbs effective rate which would be the same effective rate as running a 40.5mm bar.
Obviously these percentages and numbers are not correct vs. the real 27mm-29mm bar, but I wanted to explain how the measurements became rated in mm as opposed to effective rates. The better suspension companies will tell you what the percentage increase is, what the effective rates are, and what the equivalent bar diameter would be as well...If not, the math is relatively simple given a percentage and base effective rate.
Does this help make it clear as mud? Swaybar math can be really fuzzy since I've run across manufacturers (*cough* Perrin *cough*) that don't do it right...I actually went into some math on this very issue in my Evo member journal if you'd like to see more concrete numbers behind how diameters are arrived at on adjustable bars.
