Perhaps it's part of persuading you that we have a "turn up and go", "Metro" service?
I suppose this is almost the case at peak times - it's not really critical to turn up for a particular train.
It would probably suit them to do away with timetables, as then you can't be late! They pulled a similar fast one by adding several minutes to inbound arrival times at London Bridge - hence the trains can't be late anymore and they don't have to offer season ticket discounts. Simple, but effective...