Custom Heart Rate workout vs Default Cardio give different results.

I have a Fenix 6 and am using it while doing water aerobics.  My exercise is really just jogging in place in chest deep water for about 30 minutes, trying to get my heart rate up.  I created a custom workout that just measures heart rate and time.  I've also used the default  "cardio" workout to do the same thing which seemingly measures heart rate, time, calories burned and a few other details.

When I've done workouts using both, I can average about 225 calories burned with an average heart rate of 105 using my custom heart rate measurement.  On the cardio workout, it appears to measure my HR at 170+ saying that I burned 400+ calories.  I can't figure out what the difference is between the two workouts.

Has anyone else experienced this?  Thanks for your help.