The logic is simple - the system needs to know where you began and
ended your journey so it can charge you accordingly. If it doesn't
know this, it cannot charge you accordingly, so you get hit with the
maximum fare. As TfL say (on posters, in leaflets and during regular
PA announcements) "always remember to touch-in and touch-out" (though
note that when using buses and trams you only need to touch in)

Unless you are using a tram at Wimbledon, where it all goes out the window.

Everytime I have crossed from LU to Overground with Oyster, I have ended up
being stung - what is the advice here? Touch at the interchange station so
it knows you have been there, even though you are not exiting? I just get
onto an LO train and touch out at the eventual destination - and always end
up with "Seek Assistance" and all sorts of peculiar fares and journeys in my
history. I have tried both ways, and can't work out what I am doing wrong.
Neither, evidently, can LO staff.

Actually, while I am here, can someone please explain to me why my Oyster
card regularly opens gates, but then flashes up "Seek Assistance" while I am
crossing through? If there is a problem, why does it open the gates? What
does it all mean?!

Best WIshes,
LEWIS