Air is a pretty elastic substance, it is sensitive to heat and pressure. The reason most racers go of a pressure reading is they are checking the local conditions. Like the altertude of the track they are racing at (air pressure). And also the local air temperature at the time, this affects the density of the air, also you can figure into this the amount of humidity (moisture) in the air. They all combined have an effect on engine performance through it's compression ratio (air pressure) and the amount of oxygen in the air by it's density or mass for the people that want to split hairs. So you may have to change your jets to suit the local conditions.

For these reasons (and others) that is why automotive producers have such lazy profiles on thier cams to suit a vast array of local conditions and the habits of ALL the driving public over a very large continent from sea level New York to mile high Denver. For example suppouse mr car driver lives in New York and decides he has to drive all the way to Denver...(it's a long drive I know but this is hypothetical..) He expects his car to run well all the way there and back.