I'm not sure there are enough FE'ers that come to this sight that know enough about Ham Radios in order to authoritatively give you an answer beyond a best guess.
Of course, there could be a FE equivalent of the ionosphere that you are bouncing your signal off of in order to reach other hams across a flat plane too.
There are a lot of cross theories in FE. Reasons why you might not be able to aim your antenna level may have to do with obstructions (hills and valleys, waves and weather, etc), or perhaps your radio waves are impacted by electromagnetic acceleration (the upward curvature of light across the FE, EAT attempts to explain sunsets and sunrises, and radio and light are both electromagnetic waves and would be subject to the same influences over distance) which prevent them from traversing a level path across a flat surface.