Apple watch counts half of the steps

To earn exercise or workout credit you must move at the speed of a brisk walk.

To earn 1 minute of exercise credit(green ring);

Travel a full minute at a brisk walking pace 3.3 mph or 18 minutes per mile approx.or 5.3kph or 11.3 minutes per kilometer.

Raise your heart rate to approx 75% of your maximum.

When you're at around or above this heart rate and speed for the entire minute you'll receive 1 exercise minute credit.

Source;Successful phone call I had with apple support.

The watch only records steps and distance according to the criteria above so for 50% of the time you are not meeting that criteria.