STAAD Section Properties - what is the difference btwn Zp ("distance to equal area axis") and Zm ("distance to centroid along Z")?

STAAD Section Properties - what is the difference btwn Zp ("distance to equal area axis") and Zm ("distance to centroid along Z")? Also, when I take the moment of inertia, Iy and divide it by the Zm (which I believe is the distance from bottom to the NA) or Zp, I do not get the same section modulus (labeled as Wu) about y as STAAD. It is off by over 50 cubic inches, this is about 7%, would this be just rounding? My y/u is the strong axis and z/v is the weak axis in my element.  I have a Zp of 8.7 in and a Zm of 11.7 in for this particular scenario, but usually these values are equal. Thank you, any insight is appreciated.