Nobody knows exactly why Apple supports this or that model.
But according to Mactracker, the 2017 Air and Pro had many differences.
The Air had a 5th generation "Broadwell" chip, whereas the Pro had a 7th generation "Kaby Lake" chip.
So I don't think that your stipulation that the Air had "better specs on paper" holds up.
The Pro had a newer chip, larger cache, and faster RAM (2133MHz vs 1600MHz) than the Air.
All these informations about Apple products, going back decades, is readily available in the Mactracker application (it is still regularly updated, too). I have no relation to it, just have been using this for a long time.