Answer:
The Relative Power is the total optical power in the transverse (x-y) plane and it is normalized to 1 at the Input Plane. This value decreases along z when light is scattered outside the window boundaries or absorbed in materials.
The Path Monitor uses different methods for calculating the power in the waveguide. The Power in Path method, calculates the power within the waveguide boundaries. In your case at the input plane the Path Monitor is actually giving you the confinement factor.
More description of the different methods is provided on page 266 of the OptiBPM User Reference.