I bought a zone 1-2 off-peak travelcard on a weekend from a ticket
machine at Clapham Junction, selected the Network Railcard discount
and got it for £5.

I only needed 1-2 but presume if I'd selected the 1-6 zones one it
still would have been £5?

And given that Clapham Junction is a London Overground station if I
went to the ticket office, would they sell me one for £4.90!?