I believe your $20.00 option is just the bottle, with the $40.00 option being a bottle full of LPG.
Just my take on it. Where I am it's $19.00 to fill a 4kg bottle and $25.00 to fill a 9kg bottle.
Go over the road to a Swap and Go, and pay $39.00, to swap an empty 4kg. bottle for a full one.