Yesterday morning I used Pre-Pay on the tubes for the first time during the
week (previously I've only used it at the weekends). Much to my
disappointment it seems to have charged me an incorrect fare. OK so it was
in my favour but I'd much rather it got it right.

I travelled from Blackfriars to Stepney Green at about 8:30am. As a Zone 1
to Zone 2 journey I expected to be charged £2.10 and I was. At 10:30am I
travelled from Stepney Green to Liverpool Street. As a Zone 2 to Zone 1
journey I expected to be charged £2.10 again. Instead I was charged £1.70
which is the single fare for a Zone 1 journey. My only other usage of the
card was an earlier tram journey so there doesn't seem any scope for capping
to kick in. Oyster help desk seemed as baffled as I was, though
unconcerned.

Something I've overlooked or just a flaw in the system somewhere?

G.