Dave in NC,
Carl's sail calculator is a neat tool, but I have seen errors there. Not output errors, but input errors. My understanding is that anyone can load up the specs to a boat model and run the numbers through the calculator. Sometimes the numbers come from dubious sources (like the specs listed in Yachtworld) and to the best of my knowledge there is no process in place to verify.
Then again, maybe those numbers for the SC 31 are spot on. If you're truly interested in this model, it would be worth doing some homework to verify the specs.