ASW-19 Maximum Weight

Hello. I'm looking for a sailplane with my partner and we've come across a very nice ASW-19 that we are interested in. The aircraft has a maximum pilot weight of 196 pounds, which my partner just barely squeaks under with a parachute and will probably be over with drinking water and any other baggage. After reviewing the weight and balance documents, we noticed they were using 805 pounds as the max gross weight, but the manual states 899 pounds.

Further digging turned up the type certificate data sheet, which also shows 805 pounds as the maximum weight without water. Does anyone know why the FAA certified the aircraft with a lower max gross weight than stated in the manual? How are other owners dealing with this? If we took the glider experimental, would we be able to use the maximum gross number in the manual instead of the type certificate?