Ever try to balance a highwing cabin airplane with fixed gear upside down? Just about impossible to do with any accuracy. For the same reasons thats why most plans suggest for low wing airplanes be balanced upside down. The bottom line there has to be sufficient static stability (center of mass a little below the balance points) or you are not going to be able to get an accurate reading.

Airplane configuration has a lot to do with this, Other factors are dihedral if any and midwing designs. On occassion say with a midwing the plans may suggest either right side up or inverted and that is predicated on whether or not one of the positions may allow a little static stability to makes things easier (read more accurate).

John