Great Western Coffee Shop

Was in Slough earlier today and noticed several Stagecoach double deckers outside the station, all with "South West Trains" showing on the LED display at the front. The paper sign in the window of one bus I saw was printed SLOUGH with South West Trains written in smaller type. The destinations board inside the station stated bus service to Windsor. Any reason why SWT might be providing the service to Windsor today?

According to realtimetrains the bus service was to/from Windsor and Eton Riverside (SWT station) rather than Central (GW station). I'm unsure why!

Could it be that Slough forecourt/Bus station has more space to marshal these buses than does Windsor?

I don't see how that fits.

It was replacing an FGW service, and listed in RTT as such ("GW"). SWT trains to Windsor were running.
Being hired in from Stagecoach is not unusual - they get them from wherever they are available.

So the two puzzles are the bus display and using Riverside.
Presumably the display is just what the driver thought was correct - and they may not know the reason for the hire, but may know Riverside is an SWT station. Or maybe that one was already set up and could be selected, while a better alternative would need entering by some tedious manual process.

In terms of space at Windsor, Central is next to the big coach park and I think has a bigger car park too. So maybe the work there meant the passenger entrance and internal space at the station was not available.