I’ve found that anytime tickets have been available as etickets whilst off peak you have to have paper. Same route. I’m wondering if off peak has something to do with it, and whether ticket barriers can’t recognise off peak as separate from anytime e tickets.
If you buy online they usually give the measurement of the item, plus some even ask for your height, weight etc then recommend a size. I think most people usually know roundabouts which size they need?. Obviously preference comes into it as well.