OK, I'll try again.
Let's say I own a shop, and I have 10 tanks on the shelf, 9 homebrewers walk in without tanks and each need CO2. I give them a tank full of gas and only charge for the gas, not the tank. Now I only have 1 tank, but 9 are out in the general population, but I still have 10 tanks on my books. Now happy homebrewer #10 walks in with his nice new shiny tank and swaps it for my last tank and leaves his behind. Now I still have a tank, but have 10 in the general population (10+1=11). So now I have 11 tanks on the books.
The reality is that I was riped off of 10 tanks (if no one comes back), and only have a shiny empty one to look at