That is because of the different diameters of the old and worn tyres. Trying to minimise the amount of work the diffs have to do to avoid transmission wind up.
You want the grip behind you, not in front. Have the good tyres on the front and in marginal conditions the back will loose grip and the front won't. Having better tyres on the front of my old Mondeo (front wheel drive only...) I feel was a major contributory factor in hitting a wall and rolling over. TBH I'm not sure how this applies to a 4WD vehicle.
However when I first got the Disco II, it was rather vague and wandered about the road very disturbingly, when under light throttle conditions and slightly down hill. It had the less worn tyres on the front. I did nothing other than swap front/back and the vagueness completely disappeared, noticable within in the first 100yds of driving it was that marked.
Unfortunately there is another variable in that the pairs of tyres where not from the same maker. Before the swap the front had Pirelli Scorpion Zero with a good 6+mm of tread, the rear Goodyear Wrangler HP with about 4mm.
It is considered that the shift towards understeer resulting from having the new tyres on the rear is to be preferred to the shift towards oversteer resulting from the opposite.
If you are aware of the likely effect it doesn't matter what you do.
I agree with dougal. I have been to a couple of tyre places to have a pair of tyres changed. They refused to fit the new set at the front, but would happily fit them at the rear and swap the existing rear set to the front. They gave the same reason as Dougal. Or as one chap put it, "It's to stop idiots from spinning on a roundabout in the wet!"
a) It wasn't a roundabout. b) It wasn't wet, only damp. c) I didn't spin, the car wanted to but I didn't let it. c) I wasn't going "too fast", at least not in the "idiot" meaning. d) It's a known black spot, not that I knew that before. The same corner has had the local plod off the road. If they can get caught knowing it's a "bad corner" it'll have anyone off.
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