Is Apple Watch accurate in measuring activity?
In terms of logging specific activity with the Activity app or third-party apps, I found it very accurate while using Apple Watch to train for triathlons.

For a passive activity like those you are describing, my understanding is AW is using the accelerometer together with HR measurements to determine activity and is not always accurate.  You can for example just wave your hand about to get a "stand hour" and I've noticed move data increasing while in the car. 

This is not a problem specific to Apple Watch though, all other fitness trackers I have had given similar issues. I think generally it will get better over subsequent OS releases as the AI is further trained and improved. You can dig into the detail of what it thinks has happened by looking into the Health app on iPhone, selecting the category and drilling into "show all data"