An observant friend, having read my musings on the bombings, writes:
You might note two minor things.
1. Piccadilly line going East has lots of Heathrow people on (ie lots of luggage). [So an abandoned package would not be noticed as quickly.] My guess is that bomber got off at Holborn (Russell Square has a very deep lift, as does Covent Garden) having activated the device.
2. Death toll higher than Edgware Road or Liverpool Street as blast more lethal in confined space.