That’s actually another issue which really needs sorting out. Walthamstow Cenral is also a National Rail station so the fares are not actually assuming use of the Underground. I’m afraid Walthamstow Central is quite a hornets nest of issues and you’ll sometimes be charged different fares to the one you expect by looking at the TfL fare finder. If you use my fare finder then I have added a patch to expose the different fares from stations where that is a problem. Stansted Mountfitchet is fine, but both the Airport and Bishop’s Stortford have the issue.