I have noticed of lat eon my LO travels that the displays seem be out of sync with the trains themselves - especially at Shadwell where a train is 2 mins away for a lot longer and still showing when it has left - especially at Shadwell
During my 15 minute wait at Camden Road (see above) the next train actually went backwards at one point: the countdown went something like 15, 14, 13, 12, 11, 10, 9, 11...
Just to clarify on others' earlier comments: I didn't 'cut it fine' as I didn't even know the timetable but just walked to the station at the end of an evening cricket game at Lord's. It was pure fluke that I arrived at a time that should have allowed me to catch that train.
When the light on the door button went out my hand was about an inch away and my arm wasn't even fully stretched. Unless there's a huge lag between the driver pushing his button and the system responding the driver would have been able to see me.
The display remained on 1 minute for about 20 seconds after the train started to move. Plenty of time to let 4 people get on board and still leave on time.