You get the message that you achieved your standing goal right after the hour in which you achieved it. So it's entirely possible that you stood at 22:10, went to bed at 22:30, and get the achievement message at 23:00.
A: Drop it and measure how long it takes to hit the ground.
My reason for this theory is that I sometimes bounce my leg when I'm sitting, and if I have my arm resting on said leg this sometimes triggers the stand goal for the hour.
And I say that from a position of being educated in signal processing, not just staring slackjawed at all the numbers. It's a very noisy signal, even after processing, and while it's correlated with the values you're trying to get out of the data it's not necessarily a very strong correlation. There's a lot of headwinds against getting this data out of the accelerometer. It's a minor miracle that they are as accurate as they are.
> Even if you stand all day, you still need to move around.
The purpose of the standing goal isn't just to be stood up, it's to stretch your legs and get you away from a desk (it doubles as a good reminder to take a break from your computer screen).