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
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:
19. Using a combination of tickets
You may use two or more tickets for one journey as long as together they cover the entire journey and one of the following applies:
(a) they are both Zonal Tickets unless special conditions prohibit their use in this way. The Ticket Seller will, if you ask, advise you whether you can use a Zonal Ticket in combination with another ticket.
(b) the train you are in calls at a station where you change from one ticket to another;
or
(c) one of the tickets is a Season Ticket (which for this purpose does not include Season Tickets or travel passes issued on behalf of a passenger transport executive or local authority) or a leisure travel pass, and the other ticket(s) is/ are not.
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▸ 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.