The main optical strength value in glasses corrects the focus of your eye lens. The perscription strength reflects what your furthest focus is in this manner strength of prescription = -(1÷farthest focus) for near sightedness. So if the strength of your glasses is -0.5 that means your horizon is 2 meters away without glasses.
HOWEVER, optometrists actually don't correct your vision so that you can focus at the horizon. What they do is they assume the horizon is 6 meters away. This means they will actively give you glasses that are 0.17 (1÷6) too weak (or too strong in the case of far-sighted people). That isn't a massive error, and most people don't have retinas acute enough to discern much difference between -0.5 glasses and -0.67 glasses if their true nearsightedness has a diopter of 0.67. However, I have 2.0 corrected vision (twice as acute as what's deemed "perfect vision") and so I often find myself seeing the letters at the optometrist perfectly at 6m, but when I head outside, the horizon is noticeably blurry. In fact, the last two times I've bought new glasses I've had to go back and force them to order new ones one strength higher.
Why optometrists don't automatically add -0.17 onto your prescription strength to account for the difference between a 6 meter distance and the distance to the horizon, is beyond me. Fucking incompetence in the whole optometry field.
as for the thread never heard of optometrists under-prescribing. Don't think 1/4 of a diopter matters much; I have a lot trouble deciding during the eye test on which one looks better; eye doctor gave me a stronger prescription once because I answered the wrong one and then I told it felt off.