It would appear that
FGW▸ 's booking engine is not applying the easement for Senior Railcards in the Network Area on this fare.
http://www.nationalrail.co.uk/times_fares/ticket_types/64097.aspxAlthough that says
(on Anytime fares) it should apply to the Off Peak Return (via Evesham) also. The discount is being applied correctly on FGW's booking engine to HM Forces Railcard, but not the Senior or Family.
Suggest you contact FGW to ask what's going on. In the meantime, buy from a booking engine that is applying the discount correctly. I note that
SWT▸ 's is giving the correct fare with a Senior Railcard.