Why do Indian Head pennies read differently?

[olddigger]

Variety 3 (1864-1909) IH pennies read lower and/or disc out lower than wheat & memorial cents on most if not all detectors. The Var. 3 IH and Lincoln wheat cent have the same composition (.950 copper, .050 tin & zinc), weigh the same (3.11 grams), and have the same 19 mm diameter.
The above applies for dug and non-dug coins.
Does anyone know the technical reason for the difference?