For the most part, we all live at one atmosphere of pressure. One Atmosphere is 14.7 (15) psi above a complete vacuum. Absolute pressure (PSIA) starts at complete vacuum (no gas molecules at all). Gauge Pressure (PSIG) is always indicated from one atmosphere which changes with altitude and barometric pressure. When we think of gas dissolved in beer it is necessary to think in terms of absolute (PSIA) pressure since to get all of a gas out of a liquid at normal temperatures it is necessary to expose it to a vacuum. A keg half full of beer and half full of CO2 at 0 PSIG still has 15 PSIA worth of CO2 molecules doing their thing.
The illustrations below are intended to clarify the principals discussed above. The critical point is that the correct partial pressure of CO2 is required to maintain the beer quality at least as far as CO2 content is concerned. When CO2 content changes beer quality and taste change and beer is wasted. Beer comes from the brewery perfect; whenever the CO2 content changes, quality goes down and costs go up.
One thing which is hard to show is that the gas exchange process takes place at the surface of the beer and moves down slowly through the rest of the keg or tank. As a result, most gas related problems and/or changes show up near the end of a keg. The key to diagnosing gas problems is that the problems are greatest at the end of the keg.