"Stand" measurements on Apple Watch

My watch does weird things with the Stand measurement, especially the first hour. As an example: last month I woke up at 7:45, put on my watch and unlocked it. at 7:46 I walked to the bathroom and checked the Fitness app; it showed that it was measuring my standing time. I then brushed my teeth, walked out of the bathroom and downstairs, walked over to the back door, opened it, checked something outside, closed it, locked it, and went back upstairs. I had not received a notification that I needed to stand, but I was upright and moving for at least 10 minutes out of the 14 from before 8:00. I walked into my bedroom and sat down at 8:01.

I checked the fitness app. It awarded me an hour in the Stand category -- for the period 8:00-9:00. In other words, 14 minutes of walking around (OK, including 3-4 or standing without moving around) didn't count. But about 65 seconds of moving to my bed and sitting down counted.

It is reliable that, if I get up within 10-15 minutes of the end of the hour (putting on the watch for the first time during the day), if I keep moving until sometime during the following hour, the time standing and moving around during the first hour doesn't count. But, if I get up at 5 minutes before the hour, walk around for a minute, then sit down for a minute, then get up again, it almost always gives me credit for the Stand time. If I get up, walk around, sit down, get up, walk around again, sit down again, then as long as I moved for about two minutes I get the credit -- but only if the second time I sit down is before the start of the next hour.

I assume this is the way it actually works, I just don't know why. This is the only part of the Apple Watch fitness app that works differently than I understand from the information available. Why should it not give me credit for standing, walking, and moving from the time I put it on until the first or second time I sit back down?