There is also day to day variation and measurement error. You could do the exact same test on a different day and get very different results. Much can depend on equipment calibration, liver glycogen, blood glucose and muscle glycogen levels which vary.
Perhaps we should say Estimated Fat Burned rather than Fat Burned to be technically correct. I think this is what @DominikRappaport has an issue with. Our data is not the actual fat/carbs burned but simply estimates.