Problems with fourth gear in R380

Right, the story is that the main shaft pilot bearing failed, causing fourth syncro to break. The rest of the story (and the bit I find very difficult to explain) is that all of the bearings are well worn, and the syncros are almost all worn out. So the big question is: WHY? This kind of wear surely is more consistent with a gearbox that has done 300000-400000km. Mine has only done 175000km. I've always changed the oil at 20000km as per the maintenance schedule and I've always used Land Rover MTF94. Everyone I've spoken to is at a loss to explain it. Has anyone had a similar experience or can anyone suggest any possible cause? It's mainly driven unladen but occasionally tows some very heavy loads, and has a tonne in the tray on a fairly regular basis. The only real unknown is what happened whilst it was with the previous owner (up to about 90000km). I was never able to talk to him (on account of him being deceased) but people tell me he did his own servicing and was pretty dilligent with it.