I noticed this yesterday, and was waiting for someone to mention it, but it seems to have been missed.
King's Cross is a main interconnecting point between the Tube and the National Rail. Loads of commuters come in through this station. It also has the inbound train from the Luton airport, where a lot of European flights come in to. The bomb went off on the way to this station.
Liverpool Station is also a National Rail connector, but is also the point where the Stansted Express inbound train comes to from the airport. The bomb went off just before the train reached this station.
And for Edgware Road: the bomb went off on the train headed towards Paddington Station - the spot for yet another main National Rail connector, and for the Heathrow Express train.
My thought? They all went off a few seconds/minutes early.