I'm a bit perplexed and before I abandon all trust in my HRM I'm hoping someone can give me a reasonable explanation for this.

I have a Garmin FR60 with heart rate monitor and bike sensor kit. I'm riding on a MAG trainer in my kitchen.

On Sunday I cycled 42:50 minutes, a distance of 18km, average speed of 25kmh and an average heart rate of 117. Average Cadence of 74, max of 107. My Garmin worked that out to be 784 calories.

On Wednesday I turned up the resistance a bit, I did 48:09 minutes, a distance of 11.5km, average speed of 14.4kmh and an average heart rate of 137. Average cadence 74, max of 118. It felt like harder work with a few more sprints but the Calories for this was 342.

I looked into how Garmin works and apparently for the FR60 with Heart Rate strap it's a simple heart rate/age/weight calculation.

Any ideas for the discrepancy in calories burnt?