Each service academies waiver authority is independent of the others. Also each service has different visual standards as you have mentioned in your post. You have to look at what the different services do, and what repercussions may happen to someone with a large refractive error.
The larger the refractive error, the more likely you are to have multiple other vision and occular problems. The higher the refractive error, the thinner the retina is at the back of the eye, and there is a higher likelihood of getting a retinal tear, which is an extreme medical emergency that if not fixed quickly can lead to blindness in that eye. Also if that officer looses his/her glasses or contacts, or they get destroyed, then that officer is no good to his/her troops or the mission.
Now, take a step back and look at what each service does. Air Force and Army spend the majority of the time on the ground and within a day or so of a major military treatment facility that can repair or replace the glasses. Navy, Coast Guard and Merchant Marines spend most of their time at sea and away from any major medical facility. If at sea and you loose your glasses/contacts, its going to be a while before you get another pair, or if you detach a retina it my be a while before you get to a major medical treatment facility. The result is possible blindness in the one eye. In both cases, the ship now has one useless officer who can't stand watch or provide any war-fighting or damage control assistance. Also, the sailors that this officer is in charge of are now leaderless.
DoDMERB and the waiver authorities are looking to get their investment in your education back over 20 years. So they are trying to see 24 years into the future. Obviously no one can say what is going to happen 24 years from now to any individual, but with over 200 years of medical information and statistics they try to make the best guess possible for both the individual and the service.
USMMA, USCGA and USNA all have +/- 6.00 diopters as their refractive error. Each service academies waiver authority sets the absolute limit on what they will waive for refractive error. So from the waiver standpoint, I am unable to tell you anything, as I never worked for them.
I hope this has helped you to understand some of the thought that goes into the vision standards.