CIFAR-100: What is the difference between vehicles 1 and vehicles 2? The superclasses in the CIFAR-100 dataset are mutually exclusive and all but the vehicle ones are quite well defined by its label.
Example: It is very clear why bees belong to the superclass insects and none of the other superclasses.
This appears not to be the case for the two superclasses vehicles 1 and vehicles 2: The two appear not to be clearly separable if we ignore their subclass labels.
Example: It is not clear why pickup trucks belong to the category vehicles 1 and not vehicles 2.
Clearly, vehicles 1 vs. vehicles 2 are not distinguished by size, number of wheels, number of passengers, ….
Hence, my question is: What distinguishes classes vehicles 1 and vehicles 2?
As a reminder, these are the two vehicle-classes:

vehicles 1:   bicycle, bus, motorcycle, pickup truck, train
vehicles 2:   lawn-mower, rocket, streetcar, tank, tractor
The difference between vehicles 1 and vehicles 2 is that vehicles 1 are more common and vehicles 2 are less common.

CIFAR-100 dataset super/coarse labels are not very rigorous. The CIFAR-100 official documentation points out flaws in their taxonomy:


  fruit and vegetables: apples, mushrooms, oranges, pears, sweet peppers
  large carnivores: bear, leopard, lion, tiger, wolf
  …
  Yes, I know mushrooms aren't really fruit or vegetables and bears aren't really carnivores.