You see that the circles all have their centers in other cities with airport. Airborn infantry has some kind of "proxy" tolerance for friendly airports, allowing them to go farther. While i have no real explication of this design choice, except it make airborne infantry more flexible, i'll make this speculation :

As airborn infantry is delivered to a point by helicopter, the range is designed under the restriction of this helicopter. This helicopter has a operational range dictated by the airport infrastructure, meaning it loads fuel to "go and return".

With the additional airports, the bingo point (= "empty fuel tank") may be moved, as the the next waypoint of the abstract transport helicopter will be "the closest airport".




This is a speculation, but i think it is acceptable in terms of "consistancy"

In terms of mechanism, of course, maybe it's just that the next city in range is added to the range :D. Gotta wait for germanico on this one.

Opulon already beat me to it, but yes airports.
And as far as I know the airport level influences the range of the unit. I couldn't answer this from experience however. Tend to not use alot of airborne infantry.

Hello gents,
Airmobile Infantry requires an airport as their base, and that's where the centre of the range is located. Additionally, multiple airports within Airmobile's range further increase it. The range is influenced by Airmobile's level, not level of airport.

And if that ain't enough - simply build a level 3 Outpost/Military Camp in province

In theory, so, with military outpost, we can create a "airborn" coverage that allows this kind of infantry to fly through whole continents ?

in theory yes.