There are probably no pixels with alpha >0 and <1. But in the distance several texture-pixels are averaged to form "on-screen" pixels with alpha 0.5 or similar. These are the ones, that get rendered wrong (as adaephon already pointed out).
A quick'n'dirty fix would be to change the texture filtering (GL_NEAREST in opengl). You could also use texture-shaders to reduce every alpha <1 to 0.
But there is a clean fix as well.... don't ask me how to do that though...
EDIT: ok, disregard my post xD I'm not that adept with rendering yet
There should be a way to disable alpha-blending, so that every pixel gets alpha =0 oder =1. Try that =)