I would like to add that there really is a significant difference in instructional philosophy between some agencies, and it would take a book chapter to explain the difference fully. What follows is an all too brief summary of the difference.
The most influential book on curriculum development today is
Understanding by Design by Wggins and McTigue. One aspect of this is that curriculum should identify and focus on the essential Learning of a course. Think of an archery target. Course designers should identify the essential learning in the course, the important learning, the stuff that's good to know, the stuff that's nice to know, and the stuff you really don't need to know and think of them as being in the increasingly outward circles of the target. The course must be designed to ensure that learning is prioritized in that order. The stuff in the outer ring should probably be omitted entirely.
That is in large part because of i
nterference theory. In brief, the time and effort spent learning about the stuff that you don't really need to know interferes with your ability to learn the really important stuff. This is commonly called in shorthand, "Less is more," which means that if your curriculum is overstuffed with material the student does not need to know, the student would have actually learned more in total if you had tried to teach less in the first place.
In scuba instruction, we see all of this play out in the difference between agencies that adopt what educators call a "just in time" approach as opposed to agencies that adopt a "just in case" approach. The "just in time" approach creates a course curriculum that focuses on what the student needs to be a competent and safe diver in terms of the course content, leaving out that which is not important
at that time. If the student later needs to learn other stuff, then the student can learn it at the time it is needed. In a "just in case" approach, the course teaches things that will probably not be needed, but it is required anyway, just in case the student might need to know it some time in the future. An example would be instruction in diving with tides and diving at altitude. Open Water classes usually teach them very briefly and tell students to get more knowledge if those situations arise. As an instructor teaching in Colorado, the altitude situation arises immediately, so I teach it then--I have no choice. If I were teaching in Seattle, the tide situation would arise immediately, and I would have to teach it then. If I taught my Colorado students how to dive the tides in Seattle, it would not only be a waste of time for nearly 100% of them, it would make it harder for 100% of them to learn what is really important.
So some agencies have the philosophy of teaching enough to make you a safe and competent diver at that level of training, certify you at that level, and then leave it up to you to go out and attain a high level of expertise through a combination of diving experience and further training. Other agencies teach you until they believe you have a high level of expertise and will not certify you until you are at that level. Those two philosophies appeal to different kinds of students, and students are free to select the one that fits them best.