Its a manually controlled system at Paddington!!! Someone has to post the platform to the system then auto voice takes over.
It is best practice not to advertise the platform until it has been routed in, just in case it gets changed which causes more annoyance.
If the control person is busy say on the phone, making the tea or otherwise engaged at the precise moment the train is routed into the platform there may be a delay until the magic button gets pressed.
Oh right, when we visited the control rooms and signalling centres we were lead to believe the departure screens were all automatic for
LTV▸ services, as soon as the headcode moves into the platform the departure screens are updated but there was a degree of manual control for
HSS▸ services.