I read somewhere in a book about lift and drag (by German Hoerner I think, but not quite sure) that the portion of the wing that is cut away, (i.e. the 15-20mm that would have been there if the wing was made with a sharp trailing edge) actually adds to the lift of the surface. The "effective" chord of the wing and control surface is therefore significantly larger than it actually is, when it is cut with a blunt trailing edge. This may partly be the reason why the required deflection angles are lower. (??)
Magne