Comment by michaelt

Ticketing on the UK train network is completely inscrutable.

Perhaps the A->C ticket entitles you to travel on either an air-conditioned nonstop high speed train, or a slower train that stops a lot along the way, and buying A->B and B->C tickets you can only travel on the slow train and that's cheaper.

Perhaps all the discount advanced-booking tickets leaving A have sold, mostly to people only doing A->B, and when you look up B->C there are discount advanced booking tickets still available.

Perhaps you depart A during peak hours and so pay a peak fare, but as the B->C portion of the journey starts later, it doesn't fall into peak travel hours.