Why do my players learn - slowly - offenses and defenses that we neither play nor practice?

One hypothesis I have heard is that you learn a tiny bit of an off or def when you play against it, but some say that is a forum fact.

How come my guys learn a bit of other schemes?  what determines the rate of that learning?

Its a bug. No one knows why.

they copied a key from the janitor and sneak into the gym late at night to work on flex?

Players just automatically learn other offenses/defenses by amount of playing time they receive. Doesn't really matter specifically what you play against - it's just generic - assuming you played each off/def equally; even if you didn't face a specific set all season. If you think about it, when getting ready to play an opponent, you break down their offensive/defensive sets in film and practice. This would be learning it (at least to a D/D- range IQ).

sie question - do players work better against offenses and defenses they know?

for example - if I have a player who has a C in flex - and an F in motion - will he play better defense against flex compared to motion?

same question can be applied to offense.