Yeah, unless tfl had delays (including the one's they don't bother telling you about) then a 9am train at Parsons Green would have all seats taken, some people standing, but wouldn't be rammed. 15 minutes earlier and it would be people up against the doors.
Maybe that is what they wanted to be able to leave a bomb without being noticed.
It makes me wonder if it was actually meant to go off at Parsons Green though. I don't know if it was Wimbledon to Paddington or Wimbledon to the City, but if the latter then it would get busier after Earls Court, pass Victoria and symbolically it goes to Westminster.