The high pressure gauge is only useful when all of the liquid CO2 is gone, and only gas remains. If the tank itself is still cold from filling, or in a cold room (or fridge) the pressure inside the tank is lower.
If you want to make sure you aren't being ripped off, look for the Tare Weight of the cylinder (it should be on the cylinder itself somewhere). Now weigh the whole thing on a scale, and subtract the tare weight, now you can see if they gave you 20# or ripped you off. Make sure when you weigh the cylinder you remove the regulator and such.