Tony has it right, it usually has to do with the Bending loads carried through the wing attach fitting to the fuselage.

What you are really looking for is what the Maximum Design Fuselage Weight is for the glider. This is what controls the Maximum Pilot Weight. If the wings have an extra coat of paint and filler on them you can count this extra weight as Ballast weight. Unfortunately not all manufacturer actually publish the Maximum Fuselage Weight.

Brian