[Sun Care] Why are "recommended " Sunscreen amounts so huge if nobody does it properly?

Beauty Tips For Body Care
This is something that I don't understand.Dermatologists ALWAYS say that people do not apply enough sunscreen to get the recommended SPF protection, and that they often get less than the labelled SPF (like SPF 30 when the suncreen is SPF50).My question is if nobody uses these products properly, then why not reduce the recommended amount to something that is actually practical for the average person? Sure that would lower the labelled SPF's, but it would provide a much more accurate picture of the protection that people are actually getting from their products.
Girls Blog 2015
Submitted by Kapys