Gas station lots are fairly level, as where this one was. Another factor is at colder temps the tank holds more LP. It was around 40F at the time.
Have to get used to this way of doing propane. I liked the two bottles, where you knew when the first one went empty, about how long you had before the last one went. (Not quite as long as the first one of course)
I will have to learn when I should get a fill so I don't run out at an awkward time. That's why I have been trying to figure out how all this works. They don't make it easy---1/2 does not mean half etc? Who needs that? I think I have it sort of figured out now thanks to the help here.