Divers in all organizations wind up with ID numbers. In PADI, this number changes with each cert the diver gets. However, once they achieve Divemaster, they are assigned a permanent ID number because this is the first Professional Level in PADI.
No matter how far up the ladder they go after DM, the number remains the same. IOW, a Course Director (Instructor Trainer) has the same number he/she was assigned as a DM.
Basically, it's a matter of habit to put this membership number after their designation.
Also, these numbers also are coded according to country. For example, Canadian numbers start with 2, Japan with 6, etc.
~SubMariner~