This makes sense how?

I needed to book my train ticket home, so checked on Trainline.com, the cheapest ticket was £129, which is weird as even this close to travel I'd normally be able to get a £46 or £67 ticket. A little bit of poking and prodding seemed to show it was the London to Chatham leg was the issue, no idea why, that's the high speed  commuter line and not normally an issue. Went to the local booking office instead (our local station is so noddy the ticket office is in a model railway shop), he pokes at his computer and gets the same figure, then pokes some more and says 'if I split it into two legs and do London to Chatham separately then it's £67.' So I asked if he'd had to switch to the non-high speed service to get that. 'No,' he says, 'it's exactly the same trains, but half the price, no idea why.'

Railway pricing moves in mysterious ways....