Multiple Zeros Or Poles At Origin

If a system has multiple poles or zeros at the origin, components of the phase plot may appear to disagree.

This is because the phase of a complex number is not unique; the phase of -1 could be +180 degrees or -180 or +/-540...

Likewise the phase of 1/s^2 could be +180 degrees, or -180 degrees or +/-540...

Keep this in mind when looking at the phase plots.