There are so many variables, no one can make a comparison with just two rides on different days. For starters, your power can vary considerably and you may want one bike to "win". You need to keep power constant on both rides throughout the test. Fit between the bikes can make a huge difference. Rolling resistance between tires can amount to several watts alone. Even with everything else the same, six lbs of total weight (bike plus rider) over twenty one miles definitely makes a difference (go to one of the online models and calculate the time difference.