Constructing the GNP & BR

In a thread on the Piccadilly Line board, Sladen shows in his Open Street Map the trajectory of the present Piccadilly line. Like others built at the turn of the 20th Century, the GNP & BR slavishly followed the street pattern of the roads above, as is witnessed by the twists and turns the line now has to make between the old Brompton Road station and South Kensington. Yet, when we get to Russell Square, as Sladen demonstrates, the line branches off in a north-easterley direction to take the most direct line between Russell Square and King's Cross unaffected by the road pattern above. Ordnance Survey maps from the period show the road pattern not to be very different from that of today. Does anybody know what the thinking was here as opposed to the rest of the line? Furthermore, was the policy of keeping within the street pattern actually abandoned or just thought not to be necessary? The Northern Line extensions to Morden and to East Finchley in the 1920s and 1930s and the Piccadilly to Arnos Grove in the 1930s all still seem to run beneath the main roads. It's not until the late 1930s when the Central Line extension was planned (although not opened until after the war) were the planners able to have the freedom of routing to serve Bethnal Green and then turning south-east to interchange with the District at Mile End and then north-east to interchange with the British Railways line at Stratford.