The underground in London was planned many years ago, Since then a lot of the trains have been updated, and some of the newer model ones are narrower that the old ones. So a notice was put on the platform, reminding people to be careful when getting on or off the train. The PA announcement was added later, probably after a couple of people did not mind the gap, and were injured. However the PA announcement is most probably played only when a narrower train reaches the station, but the notice on the station remains.