Karl,

I really believe this is just reflective of the manufacturer's preference to compose some simple weight and balance rules, rather than trust the unwashed and unlearned pilot to do any computations. Unless you ballast the wings, the loading rule is simple: something like 70kg minimum (depends on the individual aircraft) on the front seat , and a max of 110 kg on either seat. Easy to remember. Not totally representative of the physics involved.

Of course this leads to some strange situations. If you have a 110kg guy in the back and a 69 kg person in the front, the glider isn't legal to fly (ignoring the front seat trim weights). But add 1 more kg in the front, and you can put something like 8 liters in the tail!

Some other German manufacturers like this same approach -- rules instead of computations.