The dealer gave you a full 20 lbs of propane which means the tank was at 80% liquid volume.
If you buy an exchange cylinder the exchange companies only fill them with 15 lb of propane. They do this purely for economic reasons, they have additional expenses associated with transporting cylinders, recertifying cylinders over 12 years old, painting, etc. ... in order to remain competitive with dealers who refill tanks they put less propane in each cylinder. This is essentially the dealer's comment about Home Depot not filling tanks to capacity.