Three possibilities:

1. One station is radiating more power than the other in the necessary
direction. This means elevation as well as azimuth angle. You may both
be applying 100 watts to an efficient antenna, but one might be
radiating a lot more than the other in the necessary direction. If both
sites have equal noise levels, the one radiating less power in the
necessary direction will have the weaker signal at the other end. This
is probably the most likely reason.

2. One station has a greater noise level. This might be because of
actually greater noise reaching his QTH, or it might be because the
other station's antenna has nulls in the directions where a substantial
amount of noise is coming from.

3. Unreliable reporting. You should take all signal reports with a very
big grain of salt. Different people have very different criteria for
what constitutes an S9, or even R5, signal.

All these can lead to dramatically different reports at the two ends of
the path, with no need for one-way propagation or violation of the
reciprocity principle.

Roy Lewallen, W7EL