And given the intended use of a mortgage related appraisal, that makes perfect sense. Who would want to make a 30 year loan on a property with only 20 years of remaining life? It is a legitimate risk concern, and evaluating risk is the very reason for ordering the appraisal in the first place.
Reporting a remaining economic life of less than 30 years is an issue for a lender, and it is a conclusion that is likely to be challenged. So, if one is going to report such, it would be very prudent to have derived that number actually using recognized methods and techniques - and that does not include, "it was just an estimate."
There are many appraisers who do not know (were not trained?) how to actually extract and support the effective age and remaining economic life of a property.