If the c02 tank was in the fridge, and is now out, the high pressure gauge will read much higher. Could that be the case?
The higher pressure gauge is not like a fuel gauge at all, and I often state that I think the best way to handle them is to cover them with duct tape. It doesn't read how much gas you have left- it measures the pressure, in psi, of the c02. C02 is a liquid, dispensed as a gas, just like a propane tank for your grill.
The only way to know how much is actually in a c02 tank is to weigh it. The tare weight is stamped on the outside of the tank, and a 5# tank holds 5 more pounds than the tare weight.