It’s usually non racing stuff like demos and parades that upset the proposed timetable. Then sometimes there’ll be a few minutes ‘tweak’ somewhere that will require a new version, and very occasionally a couple of races could swap places. Even something like a change of time for one drivers briefing requires a new version.
So you get the PA approach, announce very early and risk changes, or get the opposite (no names
) where it’s not apparent until the last minute. And still get changes.