Great Western Coffee Shop

What discretion are train staff allowed in the tickets they sell on the train? And why? My question is prompted by a recent journey from Severn Tunnel Junction to Temple Meads. I arrived at the station with no time to buy a ticket from the machine (in fact I didn't even notice if there is a machine but I presume there is). The train was pulling in to the platform as I got there so I got on and when the guard/conductor/inspector/train manager/whatever his title is came round, asked for a single from STJ to BRI. I'd made the journey out there by other means so hadn't bought a ticket that morning. His machine had just run out of card so he had to go off to reload it, and when he got back to me he said, "I'll do the same for you as I've done for this lady, I'll sell you a ticket from Patchway." I hadn't expected this and certainly hadn't asked for it!

It saved me something like ^2.50 but my question is, why would the guard(/etc) do this? Perhaps he was simply in a good mood and wished to be generous to people? Maybe it was actually part of a subtle GWR marketing drive to build up "good will"?! Could there be some statistical reason? Or possibly I met a dissatisfied employee who relished the thought of depriving his employer of a few pounds!

 ???