Trains may operate to the publicly advertised times. There used to be a note in WTTs to say just that.
In this case there is pathing time in many Reading line schedules for freight trains in front, that may or may not exist on the day. By advertising the trains earlier they no longer have to wait at Southall if there is no freight ahead.
On the other side of the coin if you amend the WTT you can no longer fit a freight train path in, and the GW relief line has quite a bit of freight.