Woops, it is. Don't know how I f*cked that up.
I don't think it's any scripts that control that. Maybe there's a script or something that designates these special areas so they can be listed and used in vehiclemodelsets.meta, but other than that I'm sure that it does control special spawns to some degree. Or something else is since even after messing with that the problem still persists.
The only difference between those sections in World of Variety and the default game, is that the default game has the probability set to 1.0 for both POLICE_HELI and AMBULANCE_HELI, instead of 2. And in the default file POLICE_HELI also lists the Buzzard. Like I said though, restoring those two parts to their default values doesn't fix the issue of cars spawning up there, so there has to be some other modified file that the mod uses that causes this, but I can't figure it out. Such an odd issue.
Its probably a typo that messed up the file. It happened to me before when I edited the loadouts.meta and popgroups files. One typo, one misplaced / or a missing </item> will glitch out the whole file. I would check from the top of the file to above those lines to see if there is an error.
Also, the probabilities values are from 0 to 1, they represent percentages basically.
Also, like CP said, you can add customized vehicles to the traffic with this file. Cars like the Penumbra, Schwartzer, Youga, and Lowriders that have a vast ammount of customization are perfect to add to traffic. Lowriders however are not THAT customizable as you might think. For instance, all the exclusive Bennys mods will not be available as they require new extra fields that are not defined in vehiclemodelsets. For example, all mods related to interiors, liveries (the liveries from the new vehicles are somehow modelled into the vehicle, they are not just textures files like the used to be), bennys wheels (unless they created a new type of wheel for vehicles.meta) they are not available aswell. All in all, you can customize lowriders pretty much the same as their stock models (for example the Buccaneer custom is as customizable as the stock one, and so on).
Talking about cars, there are some vehicles that have unused liveries. Examples are the Boxville (with the cones in the front end) has one unused livery thats located in the x64e directory, but since this vehicle was updated later, its modded files are located in the dlc files, and that version doesnt have said texture. The same happens with the Shuttle Bus.
And finally, I wonder why nobody fixed the JB700 by removing those ugly ass weapons (I know theres an old version in GTAVMods, but the colissions are way off). I mean, Its just a beautiful car but those weapons just murdered it for me.