When I Travel The World Did you advise BA of the ground travel time between PHL and BOS? there are specific notes in their calculator to this effect
Given this, what date and time did you arrive at BOS? I make the road journey anywhere between 5:30 to 6:20. ive updated with a reasonable estimate and get this:
I've assumed 2 pax and the calc is now out of date and has the old figures in Euros but you get the drift I'm sure. Id ask them why they haven't taken PHL-BOS into account.