It all depends on what the loss model takes into account.
The propagation pattern of the ground antenna is usually not omnidirectional. Usually the maximum gain of the antenna is found a couple degrees above the horizon. If the model takes that into account, this would explain the observed behaviour.
Another thing to take into account is that the earth is not flat; at low altitudes you are behind the horizon which results in the signal strength diminishing rather quickly, depending on the radio frequency used.