Usually propane tanks are only filled to about 80%, to allow for expansion. That being said, it is possible to fill them to close to 100% although it is not a good idea. But this could be the reason for odd fuel numbers.
I thought the same thing...so I asked the guy who'd discovered the "fill vertically" sticker on the tank, and he said, "They're sized so that 80% full is 5 gallons. The vent [the one that starts spewing to indicate "full"] is set that way."
I found a couple of sources on the Web that confirms "80% full is 5 gal"...and one that says "Full = 80% of 5 gal = 4 gal"...
The Wave 3 uses 1/8 pound per hour on high. So eight hours on high uses one gallon.
You mean, 8 hours uses one pound, right? A gallon of propane (liquid) weighs about 4 pounds.
[update- I had written "5 pounds"...for some reason, but corrected it to "4"]
This solves your problem and is the direction I went.
Link
Home Skillet went the same way in his custom build.