The accounting argument falls a little short.
The actual additional cost of transport by air was gross (collective) weight, not cover count ... the stamp count meant even less.
Stamp sales signal intent, but they do not determine usage and, of course, all sorts of stamps could frank a cover sent by air.
While the rate needed to be set to cover the cost, the occasional use of purposed stamps contributed nothing to the accounting.
My own thought is that 'airmail' stamps made manual sorting/handling of the mail a bit faster.
https://goscf.com/t/47132 ... Parcel Post Stamps on First Class Mail
Cheers,
/s/ ikeyPikey