I think it's obvious what is going on here... It's a combination of driver and conditions. With very little of it being the vehicle.
I can drive my 4runner down a trail and be followed by a very similar vehicle.. If he just moves over a few inches.. He can become stuck while I just breeze right though it. The reasons this can happen are obvious. Those few inches mean a completely different contact path with the ground. Inches can make a huge difference.
Also, If there is long slick stretch/section that you drive regularly... You learn where to keep up momentum....and where to slow down. The driver can make all the difference in making it look easy. The vehicle has a big part also, but the driver is at least 70% of it.
Another scenario. Going down a creek bed and encounter a small waterfall... First one up so you walk right up it with decent traction, but deposit a little mud... Few more guys struggle and bring along more mud... and by the time the last guy gets there it is impossible no matter what he is driving...
I don't have much experience in snow (since we don't have it around here... But I've driven trails on the same day where they don't even require 4wd when dry, but become scary as hell when wet... conditions are constantly changing in inclement weather. Ice is a total wild card.. Unless you are running studded tires.. you have to just go along for the ride and see what it will let you do.... No 4wd system will help much.