[n_plus_1_acc]

They are possibly referring to this:

>Journeys to or from Heathrow Airport are priced at a premium due to using the rail tunnel between the airport and Hayes & Harlington. That stretch of line is not part of the Network Rail system but owned by Heathrow Airport Holdings, who charge TfL an additional fee for each train that uses it. Heathrow is nevertheless included within the Travelcard scheme and daily/weekly fare capping as a fare zone 6 station.

https://en.m.wikipedia.org/wiki/Elizabeth_line

[Symbiote]

I have now found it on [1]. Yesterday this was showing £5.60 for both options, but now it shows £13.30 for the Elizabeth Line.

I think it's most likely TfL's annoying language/translation plugin (I'm not in Britain now) was messing something up, and the fare wasn't updating.

[1] https://tfl.gov.uk/fares/find-fares/tube-and-rail-fares/sing...