At some times, activator rails behave "normally" and eject mobs and players to the same block, diagonal to the rail, every time, regardless of if there is a place for the mob/player to land. At other times, ejection is inconsistent. Sometimes the mob/player will be ejected to the diagonal block, while other times it will be placed on the rail. Being placed on a rail can even happen when blocks are above the rail. Also, whether or not a mob/player can be ejected with a block underneath them seems to fluctuate. There are some instances where the mob/player will simply not be ejected. I can not identify exactly what causes these inconsistencies. I originally thought they were location based, but locations which exhibited one behavior, changed behavior later. I also noticed that when mobs/players where ejected regardless of whether there was a block to stand on, those locations seemed to be more consistent. This might have been coincidence, though.