The vehicle constrains are indicated in the vehicle SIM.CFG file, with an expression. They are usually controlled against the airplane weight, but not all of them includes such constrains.
Which means, you can edit the file yourself, the line that checks the weight should be very obvious to find.
We'll have a look to set it ourselves, because not all of them have been set, so the have an equal (random) chance to be selected and, with the latest added vehicles, we wanted to give users the ability to see them...some users always fly always the same planes, so they might think GSX has way less vehicles than it really has...
