First it's important to note the standing time does not mean you stood for an hour, it means you 'stood up' for at least a minute within that hour.
Also, it doesn't appear as though the standing sensor is very accurate. For example, if your watch arm is at a 90 degree angle, horizontal to the floor it does not think you are standing.
But if your watch arm is pointing toward the floor the watch considers you are "standing" (you actually do not need to be standing, it's all in relation to the position of your watch arm only)