Is the LSI actually an "Index"?

A listener emailed the old ruleyourpool email address (the new one is ruleyourpool@gmail.com) and brought up something interesting. He said "Nowhere in your RYP podcasts have I heard you mention that the LSI is not an index. It's a delta between the saturation pH and the current pH. LSI is the difference between the pH you have and the pH your water wants." Let's discuss this.