SongMan,
The biggest difference between the flap extension angle with the Piper and the Cessna isn't so much as High wing vs. Low wing, but its a difference in the flap design. The Cessna utilizes Fowler flaps while the Piper has plain flaps. Flowler flaps change the camber and chord of the wing, while increasing surface area. This dramatically increases lift while keeping drag minimal. The Piper on the other hand has a much simpler flap system that simply swings down on a hinge. The flaps are much smaller, less effective, and therefore, require more angle of attack. That would be the best answer.
As far as the fuel is concerned, the reason you have a difference between night and day is primarily due to fuel expansion. When fuel cools, it shrinks. When fuel heats up, it expands. This is why a Cessna has fuel vents on it, so excess fuel can drip out if fuel expands beyonds the confines of tank itself.
However, don't think that you are "losing" fuel. The same BTU's of energy exist within that tank whether it's cold and reading 16 gallons on the dipstick, or hot and reading 18 gallons on the dipsticl.