Because they are different devices. 375 is simply a reference point and it is relative only to the device that is measuring it.
Paka -- Now you know I've measured both of these devices, and when they say 375°F, they both mean it. Both are really true to the temp when the bowl is ready. (IIRC it was measured within a few degrees.)
*NOW* what happens DURNING draw however... The Ascent's battery and portability(small size) deny it the ability to heat room temp air as rapidly as one draws. This is why it is suggested to let it recover between draws. The curve plummets on draw and recovers back to set temp. The Herbie on the other hand, is designed to keep the temp precise on point. When I put it on load, the curve is essentially flat.
But the rest of your points are right on mark. Vapor and air mix between the two are quite different (Herbie seems to like a very loosely half-filled bowl, and the Ascent likes a denser pack.). And good point trying to debunk the thickness myth.