"Luis Ortega" wrote

[edge of film] the bottom of most negative frames ... ~1mm lighter band
... looks fogged. [why?]

One cause is the edge of the film not getting developed
because it is up tight against the spiral in the processing
tank. The stripe is unfixed film, showing up light on the
contact sheet.

If the stripe is black on the negative then:

With 120/220 it is normal for some light to get to
the edge of the film. The fit of the film in the spool
is an ooch loose, leaving a gap between the spool ends and
the roll of the film - light gets in and gets to the
edge of the film.

If the film is really loose on the spool, Holga & Co.,
then the fogging can extend in quite a ways.

If this is a 35mm roll then it may be because of: a leak
in the cassette; film was bulk loaded and some light got
at the pancake of film in the loader; mechanical stress
of loading the film on the developing reel caused fogging.

And sometimes 'just because'.