Let me begin by saying that the calorie count on the cardio machine is EXTREMELY INACCURATE. This is unfortunately one of the greatest mistakes people can make when people are divising their diet and exercise plan. The number of calories you burn during the exercise will greatly depend upon your own unique genetic makeup. The machine has simply thrown a few numbers in there to whip up a score with very little absolute relevance to your workout.
That said, the calorie count can be a great way to monitor your progress on a machine in a relative fashion. For example, if the machine indicates 246 calories after your 15 minute HIIT workout, aim for 250 calories next time. You can use this reading in a similar fashion to the distance readings - but only use it to guage your progress and NOT as an indication of the amount of energy used or thus fat burnt.
As I have examined in one of my more recent articles, "The Fat Burning Zone", you will see that the whole point of performing HIIT is to stimulate your metabolism and burn calories for around about the 24 hours following your workout. The amount of energy extracted from fat during your workout is completely insignificant relative to the amount utilised post-cardio. I very highly recommend you have a good read of this article.