As i think gotbeer pointed out elsewhere, they checked for calories in urine but mention nothing about testing breath sweat and fecal matter for lost calories.

Also, something that just interests me, is the fact that they collected the same amount of urine from both twins. i have found that most people who have lowered their carb intake, especially if they are following the usual guidelines and drinking plenty of water, tend to have more urine output and more frequent bathroom trips. i don't know if this means the lc twin possibly was not drinking enough or if the non-lc twin was drinking a lot as well.

Lastly, i agree with adkpam, in that i think a portion of the 9cal/gram of fat is used up changing that fat to fuel