Component height is an important component property that is deeply attached to it. In earlier versions of ODB++ this property was just a number without unit. Since PCB-Investigator needs a unit for certain features like the 3D visualisation it assumes the unit "mils" by default. The Change Component Heights dialog which can be reached by Developer » Component Heights is a fail-safe that allows you to change the unit if it is falsely assumed by the program. Please note that this option only affects the component height property.