There are several theories about the difference between car and caravan wheel fixings, and as far as I know there hasn't been a definitive explanation. To be fair there haven't been many recent reports, whether that's due to changes that some manufacturers made to the fixings and the advised torque settings , or whether it's down to caravanners heeding the advice and rechecking the fasteners.
It's not a foregone conclusion that trailer hubs are the same as caravan hubs
Do cars carry more weight on their wheels? I suspect that if you do the maths SA caravans may well carry more weight than any tow car wheel, which is why caravan tyres often have a higher load rating.