Some math:

.0539 Cal/watt/min + 1.23 Cal/min is the calorie burn formula used by a popular exercise equip manufacturer. The first part converts your measured power output to Cal (or Kcal). The second part is how much you burn (an average) just sitting on the bike being alive.

An ESTIMATE of the power needed for an average cyclist to be effective at riding outdoors and climbing hills is 1 watt per lb of rider/bike weight. So a 175 lb bike/rider would need to produce an average of 175 watts while riding.

Lets assume a 45 min ride:

.0539* 175 * 45 + 1.23 * 45 = 479 Cal

This is based on MEASURED watts. To make this kind of calculation based on HR is marketing.
Two people, one fit, one not. Will their HRs be the same for the same amount or power output? No.
Two people, both fit, same age, same size. Will their HR be the same for the same power output? No.
Two people, same HR, same measured power output. Both same fitness level? No. One could be trained athlete, other could be a beginner about to stroke out.
Same person, different days- will their HR be the same for the same power output? No. It can vary based on sleep, training load the days before, stress, etc.
And in all of these cases, calorie burn is based on power output.

Pick up a good book on HR zone training, and use your HR monitor to help you determine your training zones and improve fitness, and forget about watts and calories unless you have a power meter (and race, IMHO).

And no idea why your Garmin would show such a high number for that one session...

Pulled most of this from an indoor training instructor forum.