This can be an interesting one... To explain:
As you are probably aware, the '20', (or '6')mark on the snellen chart indicates 20:20 vision - this is not ACTUALLY perfect, (some people can see better than this), but is the perfect 'standard' that we strive to achieve.
Correction, however, may depend on the individual. To quote a case, take 2 people, both with 20/200 vision: (You would normally find correction for this around -3 to -4 prescription-wise). BUT, one person may respond to a -2 for required correction, and the other person require a -5 to achieve the same clarity. (I have personally seen evidence of this, but it may not be the norm - if anyone knows differently, please feel free to correct me).
I have found that the reason is usually down to factors such as previous prescriptions. For example, a person who is 'monocularly challenged' (could have 20:20 in one eye, and 20/200 in the other), may have gotten along for years without correction, the brain will compensate for the weak eye; when they DO decide to correct the weak eye, (for whatever reason), they usually find that a much weaker dioptre strength will be required for clarity. Another person, (20/200 in both eyes, and so NEEDING the correction at a much earlier age) - THEY CAN'T SEE! - who's occular system has become accustomed to repeatedly stronger prescriptions, will normally require much stronger lenses to achieve that same clarity.
Unfortunately, it appears to be the case of: "if you don't use it, you will lose it", with perhaps a better option being some form of vision training, (such as the Bates method).
FD