Did you model a rough opening height for your window that has say a sloping sill and now your head height is not accurate in your schedule? This is why.
We've recently discovered that the Head Height instance parameter in a door or window family is calculated from Sill Height + Height (easy right? wrong). It's not the Default Sill Height parameter as you would expect but rather the lowest point of your geometry that defines the Sill Height Revit uses to calculate Head Height (Head Height = 2'-8" + 4'-0" but should be 3'-0" + 4'-0") .