I want to get from town A to town z.
I could go from station A1, that then stops at station A2 before continuing to town Z.
The cost of the ticket is the same from A1 to Z (via A2) as it is from A2 to Z.
I live an equal distamce from stations A1 and A2. I want to buy an Advance ticket as it will cost me a quarter of the price of buying one on the day.
But I cannot buy a ticket from A1 and get on at A2, even though the train stops at A2.
Bloody barking and unnecessarily restrictive.