Why do so many people receive ever increasing eyeglass prescriptions? Perhaps it's due to repeated, technically accurate, lens prescriptions during childhood.
Different body parts grow and change at different rates, at different times. I suspect the resulting changes in visual acuity, if left alone or moderately corrected, would tend to self-correct over time, or, at least, they would usually not worsen at a rapid rate.
In my experience, optometric prescriptions are set to achieve the best possible vision under unrealistic test conditions.
The usual eye test is in a darkened room. The patient is shown a Snellen eye chart of letters, simulated to be about 20 feet away. Various lenses are selected for the patient to look through, using a phoropter machine. With each lens change there are questions such as, "Which looks better, this or that?"
This routine is used to determine an overall lens curvature, measured in diopters. Next, choices are made about relative astigmatism, which is a difference of focus in different radial planes. The measurement results in a cylinder of 0-180 degree of varying diopter strengths. If the idea of cylinder is not clear, imagine looking sideways through a perfectly clear glass cylinder rod, like a clear swizzle-stick, which stretches an image in one direction.
Another chart is presented at book-reading length to determine the ability of eyes to focus on things close.
The Snellen eye chart displays rows of letters of varying sizes, indicative of relative visual acuity. The 20/20 line indicates the ability to discern, at 20 feet, what the average person would see at 20 feet. There are other lines indicating superior than average acuity. Under the test conditions, you might be able to achieve better than 20/20, with the right lenses applied. Maybe you can get down to 20/15, or even 20/10, especially when the patient is trying very hard to stay focused; "Which is crisper, and more clear? Is it this, or that?"
Perhaps there are some problems with the optometric prescription process. I think there is some controversy about this.
The testing is done in a darkened room. Eyes naturally dilate in a darkened room. It is also possible a hurried optometrist applies eye-dilating drops in advance of the vision testing. They do that so they can easily examine the insides of the eyes, using a rather bright slit-lamp. One's pupils tend to constrict under bright light, and a dilated pupil makes it easy to get a good look inside.
Eyes focus more clearly when the pupils constrict, which is unlikely in a darkened room, possibly with drugs administered that prevent normal pupillary action. Constriction of the pupils creates a pinhole effect, making images sharper. Dilation makes images less sharp. Some may know the "blurry eyed feeling" coming away from the optometrist.
Testing in a darkened room with dilated pupils is quite different from average real life scenarios. I think all this means the testing is simply a non-realistic scenario.
If you're in a dark room and need to see, what's the first thing you do?
It's not unusual to hear people say they were prescribed a series of ever increasing eyeglass prescriptions. Over time, the eyes adjust to the lenses, rather than adjusting to the natural non-lens environment. I think repeated re-prescription for young people interferes with what would be a more natural growth process.
This might be interesting to parents with children.
(May tick off any one who practices this, or possibly add to their thoughtfulness.)
Go out on a sunny day, and see just as well, or better, than people who say they've been told, or decided, they do or don't need glasses.
Part of the point there, is that if you get the "extra perfect" prescription, tuned for a darkened room, and spend a lot of time out in broad sunlight, or reading material up close, then your eyes and body train to adjust for that, and that, I think, can lead to ever increasing prescriptions for young people when they, and their eyes, are still growing.
PS A little looking around turns up a lot of material that I think confirms at least one of my suspicions. Testing eyes focus in a darkened room, possibly with use of drugs that dilate the eye, does not result in a realistic best prescription.
The person with the major, -7+, coke bottle lenses was likely the geek whose parents were overly fussy, took them to the optometrist every year, and made sure they got into reading books all the time, using those lenses designed for the best possible acuity. The essence of this is simple, their eyes were trained into myopia, by the continuous prescription and application of lenses.
"Glasses" really are spectacles if you're Elton John.