Ok,i just solved what causes this distortion.
If i export the model from blender with Flat Normals steep parallax works perfectly,if i set normals to “Smooth” than the distortion reappears.
I have no idea why this happens,but this is what causes the artifacts (i tried with other models too).
Sounds like maybe your normals (when smoothed) point too far away from each other which reduces the accuracy of fragment interpolation. ie: not enough triangles defining the real shape.
So do you think subdividing the mesh will make the trick? 
Maybe.
Ok,i subdivided and smoothed normals and vertex and this is the result:
http://s12.postimg.org/vbr2q3tq5/background22.png
As you can see the artifact is minimum 
Thanks everyone!