Could it be the tax assumptions? cFiresim doesn't have you to enter your expected tax rate, so it's calculating it based on your inputs of spending as income. Rich, Dead, or Broke has you enter in an Average Tax rate over 30+ years of retirement. Many of us try to minimize our tax burden, so I'm sure I'm entering an Average tax that is not quite right.

If you enter your details into cFiresim, using investigative options to calculate the "Maximum Initial Yearly Spending For 100% Success

Minimum Success Rate", then enter the value returned into Rich, Dead, Broke with a 0% tax rate, your results should be under .5% different.