Frequency rather than timetable
On metro lines and most tram lines, the operator does not publish an exact time for each train. It describes a headway instead: one train every two to four minutes at peak, every eight to ten minutes in the evening. That interval is what the dataset describes, and therefore what structures the display.
In practice this means a two-minute gap between the announced time and the actual one is not an error on such lines, it is how the mode works. Conversely, on a river or sea shuttle the timetable is firm, and the same gap becomes significant.
