I am not sure if this is a South African thing, but I understand that the towing limits are set by the mass the vehicle can pull off on a certain grade of hill (assuming coupled brakes!). I am sure there are other factors that come into it.
Thus even though my landy battles to wheeze its way up hills in forth gear (SerIII Nissan LD 28), slot it into low range and I can move off with just about anything on most grades, hence the high tow weight.
Most 1ton pickups here (Isuzu, Hilux etc) have a combined mass of about
3500kgs, whereas the early 110's had a mass of 7050Kg or somthing as the max coupled mass. However I recently came down from Johannesburg to Durban via Lesotho, a bit of a diversion, and having hit a 19% grade at 3200m above sea level, I nearly had to change into low range just to get the relatively empty landy up the hill. If I had a 2 ton trailer behind I am not sure if even Low 1st would have got it up.
Regards Stephen