Scientific American - Mercury Messenger Mission

I got my March 2015 issue of Scientific American & started reading right away. On page 24 was a 1 page article entitled "Ode to the Messenger". It discussed the end of the Messenger mission to Mercury. In the article they mention "Weird Magnetic Field - In the 1970's Mariner 10's flyby revealed that Mercury has a magnetic field. Yet Messenger's observations showed that, strangely, the field is not centered inside the planet but instead is offset towars its north pole. Theorist have yet to explain what causes this asymmetry. "
I shared this with my son and we both thought it odd. A magnetic field is created when a solid metal core, such as Earth's nickle core, turns at a much faster rate than the molten area and the crust. This difference in spin creates a magnetic field. So as far as current thinking, in order to form a magnetic field the core must be centered and solid and round and a form or magnetized metal. Mercury does not have that setup. So my son put forth his theory,  what if the core of Mercury was cylinder, a long cylinder metal "rod" or shaft that turns at a higher rate than the molten core. This would make sense with our current understanding of Mercury.