Large discrepancy in Calorie Calculation between Forerunner 965 and Descent G1 ?

Large discrepancy in Calorie Calculation between Forerunner 965 and Descent G1

 

Forerunner 965

Pool Swim

1300 m

51:43 min

3:55 /100m

117 ave hr

422 calories

 

Descent G1 Solar

Pool Swim

1300 m

50:58 min

3:55 /100m

127 ave hr

208 Calories

 

Similar times, same distance, but even though my Ave HR was higher with the Descent G1 swim it was less than have the calories burned than with the FR 965.

Seems odd ?