Jet Hopper Differential Rear Axle

[SuperBandit]

Hi all,
After the SuperBandit truck, I started my own collection of Tayio cars and got myself 4 Jet hoppers.
Two are the old classical red, one is a yellow heads up hopper and the last one is a grey Jet hopper.
Of all the four, only the grey one is in running conditions. The thing that struck me is that the rear axle has got a differential! If you spin one wheel with your hand the other one will spin in the opposite direction.

I guess it must be one of the latest being produced because all the others (the super bandit included) do not have the differential.

Can anyone shed any light on this?

Thanks.

gio.

PS:I'll post some pictures of my collection later on.