I think it must rely on the position and movements of your arm. Arm hanging at your side: probably standing. Arm swinging: probably walking. Arm out on a desk or on your lap: probably sitting. Which is why I think it may have trouble with standing desks, unless it remembers that it moved upwards when I first stood and it hasn't moved down again.
My desk is easily adjustable, so although I might rack up 6 hours standing over the day, it isn't all in one go. Typically, I stand from 8:30 until around 11am, sit until lunch, go for a walk at lunch if I have time, stand again for a couple of hours after lunch, sit for hald an hour or so, and stand at the end of the working day. This gets the best of both worlds, healthwise.