Apple 4 calorie burn off by a lot

I received an Apple 4 For Christmas.

The calorie burn on exercises seems way off to the low side.

First, I've worn it during a variety of activities along with various Polar chest strap monitors. The duration and avg HR seem pretty good.

However, when I compared calorie burn (total) on AW vs Polar (which also incl age, sex, height, weight data) for HIIT on an elliptical for example (54 Min 122 avg HR) the AW says about 515 total calories and the Polar says 653.

As a tie breaker I put the data into five different online calculators. All had total calories for same age, sex, height, weight in the 650-665 range.

Calorie burn is critical for calculating macros in the diet.

Will Apple get this fixed or do I just have a very expensive heart monitor?

Disappointed.