[Piwoslaw]

Matt, I'll not question your numbers, but I can't agree with your theory.
As Pale stated before, drag increases with the square of speed. At 70mph you are going only 7.6% faster than at 65mph, but your drag is 16% higher. By increasing your speed from 65mph to 70mph, you have to overcome additional growing drag for the whole time interval that you are accelerating. Once you switch to neutral your deceleration rate is greater between 70 and 65mph than between 65 and 55mph. The extra coasting time you get isn't enough to compensate for the extra fuel that went into accelerating over 65mph to 70.
What I remember from school physics says that P&G between 55-65mph should be better than between 55-70mph, BUT every car is different and your's just might have the right acceleration at the right speed and rpm to give you the figure you got. You definately have a slightly higher average speed when doing 55-70.