Olof Lagerkvist wrote:

Yes, of course, but in the Piccadilly Line case the question is more why
the signage was changed from from northbound/southbound to
eastbound/westbound north of Holborn.

Perhaps historic as "southbound" would have been at Aldwych, but also the
general direction of travel and the actual destinations - you would describe
Russell Square and King's Cross St. Pancras as "north" of Holborn rather
than "east" but I wouldn't use "south" alone to describe even Leicester
Square & Piccadilly Circus. (And the entire strategy on the tube is to
discourage as much use of Covent Garden as possible so that wouldn't factor
either.)