I've experienced varying results using CKD's. Usually the differences have been due to changes in my approach. The 2nd time I did a CKD I got pretty poor results, but this was due in part to following Lyle McDonald's guidelines to the T rather than being somewhat loose in my adherence to them. I know of no physiological change, other than perhaps you are leaner from your first go around and your bf% is closer to that at which your body resists further fat loss. Hopefully there are some experts who will be better able to address your question.