The reason you get this is because the gauge is reading the air pressure in the tank, but the CO2 is liquid inside. So it doesn't matter how much liquid CO2 you have, the air pressure will always be the same. Every little bit of air let out of the tank allows a little bit of CO2 to go from liquid to gas. However, once you run out of liquid CO2, that's when it drops off a cliff because now you can't replace that air pressure. That's why Bobby mentioned weighing the tank, that way you can weigh the amount of liquid CO2 you have left. I'm sure someone with more know-how than I could devise a scale and weight system to measure CO2 levels (I think I saw one for cornys), but I prefer to just ignore whatever the tank gauge reads, at least until its empty and I can't pour anymore beer.