The platform numbers on the display boards don’t use decimal, or are somehow encoded with the wrong number until 30 seconds before the train heads off.
And given the doors close up to 45 seconds before departure, -15 seconds just isn’t enough time to storm up the escalator, run sideways and fall down the next escalator to the correct (and-now-empty) platform.
As a local I’ve never had the need to change at the station but with 14 (I think?) platforms you’d think they’d give a reasonable amount of time to change!