The one difference that may exist (and I don't know if it does exist) is that an independent PADI instructor can not teach an Assistant Instructor nor Instructor course. But the only NAUI course directors I know/knew were associated with dive shops so that may be a difference -- perhaps you can be a NAUI CD and not affiliated with a dive shop to teach at the instructor level.
There are many NAUI CD's that aren't shop monkeys.
OK then, if that one wasn't silly, then how about "International Diver"
Cant really explain that one for you...but its no more silly than about 90% of classes that instructors will charge you for. IMHO 90% of "Specialty" classes are money grabs, in which the student leaves with no new skillset and a plastic card that qualifies them to do nothing at all. The Hookah course is basically an OW course by the way....just with a hookah instead of a tank.
---------- Post added March 11th, 2015 at 02:56 PM ----------
I will assume you have taken a statistics course at some point in your life....So you should know how useless those numbers are. And I don't need to be a mathematician to blow holes right through their numbers.
This tidbit amused me "Disposable personal income in the United States reached an all-time high in November of 2014"......Really? Not so much.