OK, here is something for starters.

There are optical limits and neurological limits.  There are many
variations in people's optical systems.  Currently, the best way to
measure this is with something called aberrometry, which is quite a bit
more sophisticated than the methods used in the traditional eye exam.
The pupil also plays an important role in fine-tuning the optical
system.  The pupil limits the influence that optical perfections have
in distorting vision.  Saying that two people have the same
prescription is like saying that two people look the same because they
have the same height or the same weight.  It ignores the facial
features.

DrG

Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision?  For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other.  What factors
influence this?