So, you got stuck recently (possibly in the snow somewhere) and noticed that you had only one wheel per axle spinning. Well, that's normal. I know you expected more from 4WD/AWD - but spinning wheels are part of 4WD life. The various traction control (slip or spin control) systems try to prevent that as good as they can, and some are more successful than others, but bottom line is, that when conditions are really bad, you end up with spinning wheels and you are stuck. Older vehicles without traction control suffer the most.
Reason for this is mainly the low traction between rubber and snow and specifically the sometimes unequal traction on your 4 tires. 4WD/AWD is a major (100%) improvement when it comes to traction. But since traction on snow is very little, you get twice the amount of "very little" in 4WD. Usually that is enough to get you going and keep you moving.
.. ideally you need absolutely level ground for that. As soon as the surface (or material on the surface like snow) is uneven, the chances are very high that you will encounter spinning wheels (usually diagonally opposed - like right front and left rear). Again, traction control tries to control this - emphasis is on "control", it can't prevent it.Manufacturers always have the disclaimer in their brochures that as much as their systems improve driving stability - they can't defy the laws of physic.
Now you want to know why you lose traction on uneven ground. OK.
Uneven ground makes the wheels sit at different heights. The low sitting wheels (they are the ones that lose traction and spin) carry less weight. The springs on those wheels are slightly relaxed and relaxed springs indicate less weight. On visual inspection you will see that the distance between fender well and upper tire is greater than on the other tire(s). Less weight means less traction
- and since you have already so very little traction on snow, those tires start slipping and spinning.
The other two wheels carry the additional weight and get more traction, but its always the wheels with low traction that dictate how much torque can be used before traction is lost.
Here are ways to reduce the chance of slipping and spinning tires:
1 - A vehicle with (manual) differential locks - very few
have it though
2 - Good snow tires
3 - Chains
(yes, chains - even on a 4WD/AWD)