More simply could it be that with flaps extended you have a nose pitch down moment, in turn a lesser angle of attack to the relative airflow at the wingtips which in turn would produce less induced drag with the flaps down.
Remembering induced drag is the result of the pressure difference of the air at the wingtips.... air wanting to move from higher to lower pressure at the wingtips.
The flaps are producing the extra lift and drag at low airspeed but is this is not induced drag.
Not an expert, Just putting it out there.
Last edited by MerlinV8; 22nd Oct 2008 at 13:34.