Why is everyone using 4 digits for batteries that are labelled as 5 digits? 18650 are 18650, not 1865 even if it makes more sense that way. I wonder why the no

Why is everyone using 4 digits for batteries that are labelled as 5 digits? 18650 are 18650, not 1865 even if it makes more sense that way. I wonder why the nomenclature changed?