from my understanding (which i could be wrong here) HIIT cardio is another form of anaerobic exercise as opposed to standard cardio which is aerobic. during anaerobic exercise, your burning calories from different sources than just hopping on the treadmill and going the same pace.
it's the chemical/hormone release that takes place afterward too. just like weightlifting, your metabolism will be raised for a day or so.
*waits for a scientific approach to question*