The Power Spectrum graph for Observation Line 1 is your reflection. The absolute value option is available because the sign for the power measured is based on its direction relative to the normal of the observation line. The normal for these lines is the +z axis which means any power moving in the -z direction (which is the case for your observation line 2) will be negative. For people who do not want direction to be a factor, checking the absolution option simply takes the absolute value so removes the negative.
Observation Line 1 has power moving in the +z direction so there is no observable difference between the absolute setting on or off.
Scott