Hi Molly
The towball max weight allowance for the car is what the manufacturer says the car is capable of. It also assumes that the towbar is capable of taking this weight.
The caravan's max hitch weight allowance is what the chassis manufacturer says the hitch is capable of.
The car might tow other trailers than caravans that could have a stronger hitch.
Similarly the car's max authorised towing weight is what the manufacturer says the car is capable of towing (without suffering damage). It doesn't mean it can safely tow a caravan of this authorised weight, as other things need to be considered, such as stability.
The caravan manufacturer quotes its own max laden weight for the caravan based on the capability of the chassis.
As you will no doubt have read on this forum and in magazines etc, it is generally accepted that the caravan's max laden weight (MTPLM) is no more than 85% of the car's kerb weight, and it must be within the manufacturer's authorised max. (See the article on Caravan Basics that appears in Practical Caravan each month)
Keith