Hmm, if the material content itself is the same as far as learning safe diving practices, theory, principles, physics, physiology, etc. Why would the answers on the exams be different or confusing? Boyle's law is Boyle's law if the instructor communicates it effectively and the student understands it. So what if the wording is a little different. Physics is physics to one who knows it. Being confused is a sign of not understanding it or just parroting back answers.
Both of the OW courses I offer allow me to use ANY materials I see fit to get the knowledge across. There are recommended texts. But I can get SEI's OW class done using Graver's 2nd, 3rd, or 4th editions. Just need to switch the workbooks for each edition. Or use the custom workbooks I create that they not only allow but will print for me at no extra cost as long as I provide the word files.
SDI allows me to do the same and supplement the OW diver manual with other texts and supplements.
It's why my own book is now a part of the OW student materials each student gets.
As long as the student is using one agency's materials and the content is covered what makes the difference in terms of knowledge and understanding? Other than the loss of revenue to one agency when the others is used in the same class. Half the class has SSI, half the class PADI? Who cares? Makes no difference when they go to dive later as the most important card they need is MasterCard, VISA, Discover, AMEX, or good old folding cards with pics of dead presidents on them.