"Difference between being used to a tempered-pitch instrument or a non-tempered."

I'm confused. I though both guitars and pianos were well-tempered. That's what frets are for, after all (except those oddball experiments in fretting you see sometimes, one of which, the Buzz Feiten System, is used by some instrument makers). Even those other fretting system are for the purpose of spreading the error across the octave, making it possible to play in different keys without going sour in some.

Or is that not true? I know piano tuners tweak notes, but (given the Buzz Feiten System and other fretting schemes) so do guitar makers. Fiddle players can fudge a note, putting a finger-tip slightly sharp or flat of its position in a different key, I know, but the fiddle is hardly a tempered instrument, is it?

So what am I missing?

Bob