The airlock is a mini-manometer. The difference between the water level in the 2 arms of the U is the difference in pressure between the headspace and the atmosphere. If the water on the carboy side is 1/2" lower than the water on the open side the pressure difference is 1/2" WC (water column). As gas is evolved in the carboy this difference increases until the lock breaks (blurps) at which point the pressures are equalized and the levels return to being the same. So just after a blurp the pressure in the heaspace is the same as in the atmosphere. Practically speaking, a dramatic pressure change would be 1" out of 30 (mercury inches this time) after passage of a strong front i.e. about 3%. The equilibrium saturation level for the beer would then change by about 3%. Given the methods under discussion here I don't think that would be an appreciable source of error. Were a flow meter being used its reading would be corrected for temperature (effects density of gas and water vapor content) and barometric pressure (effects density).