Adaptation in energy expenditure includes both metabolic adaptation as well as "NEAT" ("non-exercise activity thermogenesis"); the latter includes subconscious changes in posture, fidgeting, and various other things that can increase/decrease the body's energy expenditure by a massive degree, in an effort to (as far as people can tell) maintain a "set point" in the body that is difficult to change. This set point resists both weight gain and weight loss, both attempting to resist the change in the first place and attempting to undo it if successful.

I'm not suggesting that it's impossible to lose weight through sufficiently large caloric restriction. I'm observing that it is not anywhere close to as simple as "CICO", because CO is heavily a function of CI, rather than the popular incorrect perception of CO being things like "exercise".