I realize it is annoying to get the stand notification when one is standing. I've receive it.
But, put yourself in the place of your Watch; how does it know you are standing? From a programmatic point of view it cannot tell the difference between standing and sitting except movement.
GPS? Most GPS decoders aren't that sensitive – their margin of error is in feet, not inches (or meters, not cm). And in my case, if I stand at my desk and my arm is hanging down it is only a couple of inches higher then when I am sitting with my hands at the keyboard. So location of the watch will not tell me that I've stood, as there is no way for the watch to tell by vertical changes that you are standing. And even if it could sense an elevation change of a few inches, how does it know you are standing at a certain height vs. sitting? You'd have to calibrate it and say "when the watch is at this elevation, I'm standing". That would only work if you never go change floor (go upstairs or downstairs).
In these threads there are hopes that developers do better than Apple at this. As a developer, I'm not sure what sensor in the watch would tell me your are standing other than the movement sensor.
Is it perfect? No. But given today's technology I don't see how it could be any different.