For me the cost of a single to home with Rail card was £17.50'ish, the ticket man said that's far too much, is your station in the London travel zones? Yes says I, then he sells me the £13.15 Travelcard. Full fare would have been £26.50'ish based on the Rail card discount, so that's how I reckoned 1/2 price at £13.25'ish for Groupsave. If I had been going to Victoria then I'm sure you're right about Groupsave being a better deal. When I say 'ish it's because I was only slightly aware of what prices were being offered and may easily have got my calculations wrong.
I guess it also depends on where you were going. Most people were happy to go to London Victoria, for which the Super Off Peak Single was £15-90, for most journeys. A handful of journeys seem to cost £29-90 for a Super Off Peak Single, but they're slower, require more changes, and are only five minutes after a cheaper train, so it would be unusual to try and get one of those. There are £27-00 tickets, but they're Anytime Singles, and are valid on exactly the same trains that the Super Off Peak Singles are valid on (at the weekends, anyway).
So I don't quite know what was going on there, unless either the ticket bod didn't know what he was doing, or there's some odd limitation on your Rail Card. As far as I can tell, on a Network Railcard or Network Gold Card, you should have been able to get a ticket for £10-50, that would have been valid on the same trains that we got (ie a Super Off Peak Single).