There is another stamp on the cylinder marked W.C followed by a number. For example one of mine has W.C 47.6 stamped on it. This is the water capacity of the cylinder in lbs. You can easily do the math to convert this to lbs of propane or you can use this chart that I found on one of the propane sites:
So, for the W.C 47.6 it says the tank will hold very nearly 20 lbs of propane. W.C. 47.8 would be 20 lbs according to this chart. That would be like a 0.4% difference and I suppose that's due to tolerance variations during manufacturing. The W.C. capacity is the allowable capacity for that particular tank. Blue Rhino might install some type of device to reduce the capacity and I would not be surprised if that were the case.