Well by "random" I mean there that the % chance of them "picking" the plane is the same as the % of the plane in the total population.
I.E Spit 16 had ~10.4% of the total fighter deaths, so there is about a 10.4% chance that whatever plane you run into will be a Spit 16.
I don't mean that someone makes a list of AH2 fighters, nails it to the wall, and throws darts at it.