The stand count is based on the Apple Watch moving in a rhythmic cycle of accelerate in one direction, decelerate to a stop, accelerate in the opposite direction, decelerate to a stop. Repeating this cycle between 30 and 60 times in a brief period of time (1 to 2 minutes) and the Apple Watch thinks you have been walking and swinging your arm, with the 2nd assumption you were standing at the time.
People that knit get tremondous stand counts, and step counts, while never leaving their chair.
Professional Chefs working in a kitchen, on their feet all day, get lousy stand and step counts, because often the Apple Watch is on the arm used to hold food items while chopping, or they are carrying something and not swinging their arms. The same holds true for people holding something while walking, or just do not swing their arms.