I have noticed this a few times, and most of those times have been with the triangle bird (last of the three for livid birds), but sometimes it happens with other weapons. When it happened today with arrow I figured this might be a large bug (possibly not looking at the status of the projectile when it is in the terrain, which would make it only a problem with really thin hills). This occurring mainly with livid birds adds to this theory because the last one travels faster, making it easier for it to "not register" hitting the terrain. The image shows what this might look like with arrow. I really hope this gets fixed because this is something that can easily determine the outcome of a game.