Many factors
I haven't read the manual, but I suspect you're dealing with several issues. For starters, boats are rated to carry more deadweight (i.e., cargo) than passengers (live weight). Deadweight doesn't walk around, upsetting balance. Deadweight should be secured against bad conditions, though. Another factor is fuel, water, and weight of outboard; these affect maximum capacity and balance. Sea and weather conditions are another factor. A heavily loaded boat will not do well in a bad sea. A very lightly loaded boat will not do well, either. Another factor that affects capacity is whether you're sailing in fresh water or sea water. Sea water weighs a little more than fresh water (1 or 2 pounds per cubic foot), so you can theoretically carry a bit more when you're sailing in the ocean than you can on a freshwater lake. Finally, manufacturer's like to be a little vague about exact numbers, because they don't want to say "X pounds", and be sued because the person put exactly "X" pounds in his boat, and sank (due to sea conditions, balance, etc.). One last thought. You can load up your boat to it's designed waterline, and sail fine. If you want, you can load it past the design waterline and still sail it (up to a point), but the manufacturer will not guarantee that the boat will sail as well, or as fast, or as safely. And no boat can do well if the weight (passengers and gear) is poorly distributed.
