I know everyone was losing sleep over this one, I've answered my own question by reading the article more carefully. There is ~ 4mm difference in diameter between the cups of the barrels of the solid 5/16" axle barrels and the rear barrels re-used for the drilled quick release axles.
Hah! I genuinely did wonder. Of course, wider cups.
I was wondering if it was a tweak to get the spoke lengths the same F32 3x / R40 4x. Which I gathered somewhere was the reason the front & rear flange heights aren’t the same (on non-dished wheels obvs).
I know everyone was losing sleep over this one, I've answered my own question by reading the article more carefully. There is ~ 4mm difference in diameter between the cups of the barrels of the solid 5/16" axle barrels and the rear barrels re-used for the drilled quick release axles.