Price of copper tubing!

Google is your friend! :p
From Campbell Hausfeld's website:

To calculate scfm, the air is expanded to a standard condition to take into account the variables of pressure, temperature and humidity: 14.7 psia (0 psig), 68 degrees Fahrenheit at 36 percent relative humidity. As a result, scfm values are larger. Campbell Hausfeld typically uses cfm values when talking about air compressors and scfm values when talking about air tools.