I don't seem to get any different results between non tracking and horizontal tracking with monthly adjustments.
The meaning of vertical vs. horizontal axis tracking is often a point of confusion in HOMER. Maybe some of that confusion is in play with you system.
Telling HOMER that your PV is on a horizontal axis with monthly adjustments means that your panels will always be facing due south (in the northern hemisphere). The monthly adjustments along this axis change the slope of the panels such that at solar noon the panels are
(roughly) perpendicular to the suns rays. During the summer months your panels will be close to horizontal (depending on your latitude) because the sun is almost directly overhead at noon. During the winter months the slope of your panels will be far greater in order to be perpendicular to the sun at noon while the sun is much lower in the sky.
Systems with no tracking are angled by default to maximize the solar insolation over the course of the year. Changing the slope monthly will increase output, but not by a great deal. The difference will be smaller the closer the system is to the equator.
PV systems that incorporate vertical axis tracking have a much larger increase in output than those with horizontal tracking because they face the Eastern horizon at sunrise and follow the sun across the sky until it sets at the Western horizon.
Dual axis trackers maintain the plane of the PV array perpendicular to the sun's rays every minute of the day.
There is a pretty good explanation of tracking variables in HOMER in the help file. Look for "tracking system" in the index for more detail.