That one is a bit odd, I can't see any obvious reason at this point. If there is a multi-hour delay BA do sometimes send out emails advising you to turn up late too. If I knew for certain that there was a delay due to the late arriving aircraft then I would risk turning up late, given I'm always HBO, but this is more problematic since they appear to have the option to depart sooner than advertised. So at ths point I would stick to arriving at the normal time, or perhaps slightly later.

One random possibility is that if this a crew change / night stopover and the new crew didn't get enough hours sleep, that can lead to odd departure delays. One I recall was when the crew were in a central town hotel and a music festival was right in front of their hotel going on until the early hours.