When a compressor pumps one CFM (cubic foot per minute), that means the intake port inhales one cubic foot of free air (air at atmospheric pressure, which is 0 psig) per minute. (Note: A CFM does not mean in any sense the compressed volume.) So the unit really measures the mass of air flowing per minute, not volume per minute, since a cubic foot of free air is a unit of mass.
This second scale has a mean and standard deviation that have been arbitrarily set. A z score, for example, is a raw score that has been transformed to a scale with a mean of 0 and a standard deviation of 1.
This is called standardizing the data. After converting, the mean of the z distribution would be 0 (because we subtracted the mean from every score) and the new standard deviation is 1. This makes sense if you think about it, because a score that was 1 standard deviation away from the mean has a z score of 1.