If you place the coil asymmetrically you are getting into the 'end effect' of the ferrite rod, so some of the magnetic field lines will escape and that will marginally reduce your inductance. It's as if you cut off a few windings from your coil. So the coil exactly in the middle has maximum inductance, moving it off to the side will reduce it very slowly until you hit the end, after that it falls off a cliff.

hth.