At different exercise intensities your body uses different percentages of energy from fat and carbohydrate sources. From one of my nutrition books the relationship is roughly: 75% from fat/25% carb @ rest (ie. watching TV), ranging to 0% fat/100% at max exertion and about 30% fat/70% carb at 70% MaxHR.

Your computer is probably using your speed to guess your energy output and HR (or VO2max) to estimate your level of intensity and therefore calculate the appproximate grams of fat you are burning in your session. If the computer is reading your HR it will be more acurate. It is really a pretty crude estimate though as it is highly dependent on your weight and fitness level.

Basically at lower intensity your body uses a higher proportion of fat than carbs BUT you use more calories at higher intensity and even though the prop of fat is lower you will generally burn more fat in higher intensity sessions. Hope that makes sense.