Air is a highly variable commodity. Sea level doesn't tell you anything. The barometric pressure at sea level in Sibera is 15.22 psi
Air is at 14.696 psi at 1 atmosphere. The weight of all of the air above you weighs 14.696 pounds.
That changes with the weather. Ignoring the weather component, the formula for pressure at altitude is complex.
The short answer is: The higher you are, the less air above you and less pressure available to the engine.
The hotter you are the molecules are spread out and there is less air available.
The higher the humidity, the less air you have because is is displaced by the water.
Running at Denver when it is hot and humid nets you considerably less horsepower then the exact same setup running in Death Valley when it is cold and dry.
This is why you need to verify the barometric pressure, temperature and humidity when you make your dyno run so that you can adjust to the conditions that exist when you run at a different pressure, temperature and humidity. If you don't match the fuel to existing conditions then you will not be running optimal horsepower for the conditions.