The why is a question best left to philosophers, might have some
success with how.

If you think of  perception as a process with selective filters you can
understand how.  Every day, all the time, with all sensory input the
higher brain centres filter the input and direct attention to the most
important bits.  Imagine you are in a crowded bus terminal, reading the
newspaper while the kid next you plays a radio.  You will attend to the
news story and ignore the radio, peoples voices, footsteps, smells and
the constant drone of the station announcements of arriving and
departing trains, due to selective filtering of inputs.  Yet you will
clearly hear the announcement of your particular bus department and
should your name be paged, you will hear it too.

Same with vision.  Your brain will filter out the blurry image of the
far eye when reading and the near eye when looking far away.  It
trained itself to do this as your vision changed in your during
cataract development.  If interested, try signing out some perceptual
psychology books from your local library.  Steven Pinker and Oliver
Saks have some interesting and readible books.

If you want more detail on the how, you will be deep into neuro
physiology and the electrical and chemical properties of brain cells.
This is beyond the knowledge base of most eye doctors.  Try posting the
question on a neuro physiology forum if one exists.

Since you understand a fresnel lens, it may explain the effect to you
if you know that a glasses lens has the same optical properties as a
fresnel lens.  It is normal to have pincushion and beer barrel effects
with lenses, the exact effect will depend on the particular power of
the individual lens.

Dr Judy

As Judy said, "why" isn't as important as "how." In physiology, the answer
to "why" is usually "natural selection."

As for "how," it helps to know that what travels from the retina to the eye
is NOT a pixel-by-pixel map of visual space. The optic nerves transmit
derivative signals, primarily about edges in vector motion. Without edges,
and without motion, the signal fades to null. If you immobilize an eye
experimentally, it stops seeing within a few seconds.

No moving edges, no information. The more diffuse and out-of-focus, the less
an image competes for attention in the cortex.

As Judy said, this is "pincushion" distortion familiar to photographers.
It's a fixed feature of convex lenses, a function of F (power), n (index),
CT (center thickness), and most importantly, VD or vertex distance - the
standoff distance from your cornea. It's minimized by small, round lenses
fit close to the eye, and goes away with contacts, where vertex=0.

-MT