Why does the label on what I've always and forever called a 20 lb propane tank now say it only holds 15 lbs of propane? Did I miss the cosmic shift in propane tank designations? I hadn't noticed a change in the cost of getting a 20 lber filled and the propane guys still refer to them as 20 lbers. What gives? Do we still have legitimate 20 lbers? I'm confused and intrigued. Thanks.