From FOSBR^▸ (Friends of Suburban Bristol Railways) :

Beware! Buying single tickets to destinations that are not on the Severn Beach Line isn't always cheaper if you buy one ticket.

Example (fares quoted are 'peak'/unresticted single fares): -
Redland - Didcot = ^35.50
Bristol TM^‡(resolve) - Didcot = ^19.20
Redland - Bristol TM = ^1.40

Some things improve, others don't:

Montpelier - Temple Meads: ^1.50
Temple Meads - Worcester (all stations): ^12.20

Montpelier - Worcester (all stations): ^21.20

That's a seven-and-a-half quid difference for the convenience of not carrying two tickets with you.

These are all single fares as quoted on the National Rail site; returns are similar.

Can anyone explain how these discrepancies happen

The SVB has its own rather unique fare structure, part of the reason for its remarkable upsurge. The other is hard work by FOSBR^▸ (Friends of Suburban Bristol Railways) and Severnside Community Rail Partnership and others in persuading the lcoal councils that it can be a successful route, if only it is given the chance. So in 2008(?) when the improved service was introduced, so the new fare structure arrived. For those who don't know, the route has two zones, with Clifton Down as the dividing point. Travel wholly within one zone costs ^1.50 single, ^2.00 return, which covers any station from Temple Meads or Severn Beach to any other station up to and including Clifton. If one travels into Clifton Down then continues, then the fare is ^2.00 single, ^3.00 return. I am guessing that if you want to go anywhere else, then the benefit of this isolationist policy is lost, and you become subject to the arcane vagaries of the national fare system.

This is nothing new. I remember some 20 years back heading from Bristol to Topsham. I knew the fare to Exeer St Davids was around ^12 then, and that it was ^1.20 return to Topsham. When I was quoted nearly ^20, I queried it. The conductor eventually admitted it was daft, when he realised it was cheaper to Barnstaple. I think I got my own way, but can't recall how.

Just one of literally hundreds, if not thousands, of fares flows where it is cheaper to split your tickets. Splitting tickets is a perfectly legitimate way to save money and I do it all the time and give advice to others. It is codified in the National Rail Conditions of Carriage and you have to abide by one of three conditions. The most common being that your service(s) must call (or you are changing at) the split point(s).

From the National Rail Conditions of Carriage:


As to why it is often cheaper to split ticket. There are many reasons. A historical one is where InterCity flows were priced at a premium compared to local flows. Another is flows where there aren't Day ticket options. Then there is the fact that some fares are regulated by government and others are left to the operator to price how they wish. Many other reasons.

There's no common reason why a split will often be cheaper than the through fare. We have a very complex fares structure in the UK^▸ (United Kingdom) and the savings from split tickets are one of the bonuses to be had from that complexity. If you ever want to find out if you can save some money on a particular trip, Red Squirrel, then feel free to post a topic on the 'Fare's Fair' board. I, or others, will be happy to look at the options.