The key is that SSI requires their instructors to be responsible for their actions. Another reason is that SSI emphasizes only those skills that will be immediately useful to the student. Also, SSI’s Advanced Diver rating requires 4 specialty courses and 25 dives so that students become truly "advanced" divers. They do have what they call "advanced adventurer", a survey of some of the popular specialties, which is what most other agencies call advanced diver. The ability to print the c-cards is nice but the machine that is sold to the shop is expensive. Most of the time, when an instructor submits the c-card info in a timely fashion, it is less than a week before the card is received. There is also a lot of support for the dive shop from SSI and other programs like Swim Schools International (a learn-to-swim program) and Scuba Rangers for kids to young to be fully certified. The 20% refers mostly to the order in which material is presented and what extras are included in the course.