The Apple Watch accelerometer ONLY detects your Apple Watch arm swinging back and forth as if you were walking with a normal arm swing.
The Apple Watch accelerometer does not know if you are standing, sitting, hanging upside down, or laying in bed, it only knows if your Apple Watch arm has been moving back and forth for 30-40 cycles in a brief interval of time.
Knitters never miss stand time and have huge step counts, while never leaving their chair.
Chefs working in a kitchen on their feet all day, miss their stands, because their Apple Watch tends to be on the non-dominate arm and is used for holding food while chopping, holding pots, pans, plates while moving around the kitchen, holding cooking pans on the stove, but rarely is their Apple Watch arm moving back and forth for 30-40 cycles in a brief interval of time.