From http://stanschreyer.com/?cat=7

"Of course, the C-Class is now most widely known because of the wing sails which all boats use. The first wing sail appeared at a C-Class championship in 1974 on Miss Nylex, and it was made of spruce and balsa. Miss Nylex won the title that year, but was defeated by a soft sail boat (Aquarius V) the following year, mostly because Aquarius V was lighter."

So the less efficient rig with greater drag from the rig beats the more efficient boat with no hull drag penalty and weight has nothing to do with it! Or maybe they weren't sailing in a tank and had a non- uniform wave pattern. Maybe like me when I go sailing I cannot optimise my hull shape for different crew weights. I have to cope with the non-optimised increase in drag. At a 10 knot average your 1kg example still gives a significant boatlength difference. let alone the opportunity for position on the water. I am sure that most sailors would not quibble about your 1kg difference but 10, 15 or even 20kg! I do however find that when I have a lighter crew/boat combination I power up earlier and once overpowered I can use rig control to reduce power and drag from the rig.