Where was that bomb going?

From the previous post

"Among the theories being examined about the bus bomb yesterday is that it may have been a deliberate tactic to cause maximum mayhem as those fleeing the tube boarded buses. Another theory is that the device went off by accident as the bomber tried to make his way to another target."

IMHO that would would be my guess too. The other three bombs were timed to explode on busy tube carriages, simultaneously. This one was almost an hour later, on a bus. If somebody wanted to maximum damage to a crowded bus I would not have expected it to be primed to go off so late - after rush hour. However, the bus was pretty packed, so if that was the intention it certainly seems to have achieved its goal.

But why one on a bus and three in the tube? Were there other devices? If that one was not meant for the bus, where was it going?