I see your point. Digging deeper,
Lockheed Report 487 addresses this issue in some detail.
Page 2
"During the maximum range flight, the following considerations apply:
a. Variation of altitude from that specified by amounts
as much as 2000' (except in the heavy load condition) has very little effect on the range.
b. With headwinds or tails winds up to 20 mph, the best airspeed is wthin 5 mph of that shown on the flight procedure curve.
c. When the wind increases with altitude, the load condition, and power conditions should be carefully considered when choosing an altitude different than that shown on the curves. No strict rules can be given covering the optimum flight procedure with varying wind gradients with altitude.
d. Increase the power output when climbing from one altitude to another. Climb at an indicated speed of 120
to 130 mph."
Figure II on Page 8 provides performance curves.. Higher airspeeds are recommended in headwind conditions.
To what degree Earhart understood or followed these recommendations is another question. We know she did not use 30° of flap for the heavy Lae takeoff as recommended on Page 2 of Report 487.