This is down to the stupid way in which Renault calculate their towing figures. The 'all other cases' figure is based on the car being absolutely fully laden with passengers and luggage - they deduct this weight from the maximum gross train weight and the result (give or take a few kilos) is what they say you can tow. In the case of the Grand Scenic, they'll be assuming 7 passengers plus luggage.
From a safety point of view, the gross train weight and the car to caravan weight ratios should be the only things of relevance. If you're prepared to sacrifice payload in the car, then you should be able to tow a heavier trailer. The problem is that the figures in the handbook are (as I understand it) legally binding, so you could be in trouble if you just hook up a heaver trailer.
I've heard that if you phone or write to
, they can supply you with a revised set of towing limits, based purely on gross train weight. This supercedes the figures in the handbook, so should allow you to tow a heavier trailer provided that the car isn't absolutely fully laden.
I'm not sure about the Grand Scenic, but some 7 seater estates have ludicrously low noseweight limits too (I've seen them as low as 35Kg.) That's another thing that you'll need to watch.