I suppose the other 3 days that aren't accounted for as official will be paid by his personal money or the Duchy of Cornwall, whilst I understand his 24/7 tax funded protection officers.
The above practice is done by businessmen when travelling, stretching company travel in destination for their personal use, i.e. rather than coming back home on Friday, stretch it to Saturday and Sunday - paying yourself, some companies charge half the cost of flight ticket if doing so.