The system can only go off the information it knows. The last time the position of the train is known by the signalling system - and thus the industry as a whole - is once it passes over Brockham LC. The next point it knows about is passing RG91 signal about a mile east of Dorking Town. What you witness is the lesser of two evils.
Either it can clear off, as it does now, a certain time offset after the train passes Brockham LC, or it can remain on the screen until the train has passed/departed Dorking Town and run for a mile. I'm sure you'd describe it as even more "farcical" if trains were lingering on the screens minutes after they'd departed, luring latecomers into thinking the train was late and they hadn't missed it.
Until the Guildford - Reigate section of the NDL is resignalled, it will stay that way.
No, I wouldn't, because this happens at many places and the impact is far less on people arriving after the train has gone - i.e. they may wait a few minutes before the right train is displayed. I would
far rather an-already departed train was still showing for a short time, than an arrived train NOT showing, and another one, with a different stopping pattern, in its place! From the
passengers' (rather than operators') perspective, the TOCs' bleatings over decades that they take accurate information so seriously consistently fail to deliver improvements.
By "Dorking Town" do you mean Dorking West? I am confused by your mention of signal RG91 being
east of "Dorking Town" (Dorking West?) and then mention of a location a mile
after the station (i.e.
west) being the next updating point.