The TTIs on these trains are always very helpful and friendly in my experience; I doubt any explanation would even be needed, but if they were in any doubt just explain the ticket machine was charging the higher price. I've never seen any TTI who was anything less than professional and polite out of numerous journeys on these lines.
(I just wish the on train experience was consistently this good in Northern-land, where the experience can be very different and inconsistent indeed)
The machine may have a strict time cut off programmed into it that would mean it would sell at peak price until 09.59 and the off peak price from 10.00?
I don't think machines can be programmed, at least not easily, to recognise when the last peak departure from the station has left?
It's not peak vs off peak; it's a question of whether you get the Railcard discount or not. The correct fare should be £4.75 from 1000 or £7.20 until 0959. There should be a warning message displaying any limitations of the TVM.
If no (or inadequate) warning messages are present and/or £12 fares are being charged, the machine is probably non compliant with consumer legislation. If this is widespread across Scotrail and people are overpaying as a result, then it's potentially
yet another case which is ripe for a class action lawsuit.
The rail industry needs to be very, VERY careful and I would urge them to seek proper legal advise in the area of contract and consumer laws if they have not recently done so, and carry out a full review to gauge compliance.