Mark, I am not a physics engineer but are you are saying that the noseweight of the van has increased by 10kg or so when you have hitched it to the van, with the stabaliser fitted, compared to when you measured it level without being being hitched up? If so how do you know, how did you measure it?
If you are simply thinking that the stabaliser is somehow taking weight from the car and adding it to the caravan, or vice versa, I am not sure how you would come to that conclusion.
When you lift the blade to lift it into the bracket I guess you place a downward pressure on the towball and when it is place in the bracket there is some force ( the weight of the stabaliser?) shared between the caravan and the car but I don't seee how this has effected the noseweight of the van to any significant effect.
Still I may be wrong and as it hurts my brain to think of these things too much, I think I'll go to the pub.