Max Gross Weight Question

Our glider has two max takeoff weights, one with water ballast and one with only pilot (134 lb difference). I just weighed the glider and the empty weight subtracted from the max takeoff weight (no water ballast) limits the useful load to 225. There's still some room to play with for the forward CG limit but it's the max weight with no water ballast that caps the pilot weight at 225. If the empty weight was lower and the CG was still within the limits, the glider manual references a max pilot weight of 245. Why is the glider legal with increased water weight but not increased pilot weight (assuming CG is still within limits)? I could see the rationale for an increased water weight gross if there was a requirement to jettison the water for landing but I could find no such restriction. So as far as I could tell, you're legal landing with water at a higher weight but not if the extra weight is pilot body mass.