I am guessing and ... searching
According to Mr Dalziel's exact words "
deduct points for cropped ears and docked tail; also for carriage of the tail over the back." it seems, that at least at the beginning, some dogs were shown undocked, thus the deduction of points for docked tail.
The standard didn't mention the tail that particular time, so it was left in the eyes of the beholder and personal preference.
In the 1878 book "British dogs their varieties, history, characteristics, breeding, management and exhibition by Hugh Dalziel, ("Corsincon")
Mr. Dalziel writes : "tail is
usually docked".
https://archive.org/stream/britishdo...e/434/mode/2up
The description of the tail in the standard seems to has been added later, but I don't know exactly when.
In the 1894 book "A history and description of the modern dogs of Great Britain and Ireland - The terriers" written by Rawdon B. Lee there is a standard with the addition of tail.
https://archive.org/stream/moderndog...e/352/mode/2up