Inaccuracy of calories count when swimming with Apple Watch

I had Power Saving Mode switched off (so all sensors was working) therefore this cannot be a reason for that issue. And as I said avg. heart rate was elevated (135), duration and distance was recorded correctly so looks like Watch knew that I am exercising just didn't calculate calories use as it should. And my details (measures, age etc.) was also correct. Still baffled!

Looks however that it was one off - I gave it a one more chance and had very similar session today (slightly slower): 36 minutes, 1,000 metres with avg. heart rate 133 and today it was calculated as 661 Active Calories and 756 Total Calories. Deffinitely improvement from 10 last Friday!!!

As this is not my first Apple Watch I already knew that and calibrated it the same way as old one as soon as purchased. I re-check recently and all of the boxes to tick are there as they suppose to. I even change unit form Kcal to J and back. I did swam with Apple Watch before (however this is my first swam since bought AW6 due to lockdowns) and never have that problem - most annoying part is that heartrate, duration, distance and styles are good (in line with my past sessions of similar duration) - just the calories count down to 10 for 1,000 metres swam is shocking. I would burn more calories walking that distance with my weight. Probably I burned more taping that response! And there is no way to re-calculate that