What is actually happening in this scenario, is one wheel on one end of the diff has good traction and it's opposite number doesn't so spins happily while it's mate that still has some bite, bites and doesn't spin. In a basic open differential gearcase there isn't anything to send the drive to both sides despite the fact that one side has grip and the other doesn't, so power being power goes in the direction of least resistance.
Next time try adding some load to the wheels on that diff to make them both spin - i.e. a couple of clicks of park brake to balance the load through the diff or a little bit of foot brake to do that same. This is what the modern electronic locking differentials are trying to do when you are in 4L or electronic lock modes, except they do this by tricking the ABS system to think that the wheels are slipping and trying to cause the ABS system to pulse and slow the spin down. It works surprisingly well, I've driven past some guys with fully kitted out locked diffs all around just by going quietly in the electronic locked mode and not digging a giant hole for myself.
Ha, @makka tag...
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