It's not clear from your message if you are referring to the Airport service vehicles that are already parked when you load a flight, or the GSX vehicles which *replace* them when you call GSX
The Airport services are assigned by airport so, clearly, it's not possible you'll see the same vehicles everywhere, unless you meant you always fly in the same airport or possibly country.
The Living world vehicles (those the wander around the airport ) are assigned by region, so their assignment is less precise.
The proper GSX vehicles are also assigned by airport but, they are many more variations, and they follow more specific rules depending on the airplane kind or parking type, and if there is more than one operator available on that airport, you'll be offered a choice.
