If a gas does work as it expands (as it does accelerating a paintball), it cools. When this cooled gas mixes with the air, the air it mixes with is cooled. If the air cools below its dewpoint (determined by humidity), the moisture will condense out.
CO2 does this to a greater extent because it cools not only by doing work, but also by evaporating from liquid to gas. When the CO2 exits the barrel, it is likely colder than the gas from a similar gun running HPA on the same day. Therefore, you're more likely on any given day to see vapor from a CO2 gun than from an HPA gun.
