Fox's symptoms started with a slight tremor, so his motor score would have been less than 10. For most patients, the "mentation, behavior and mood" scores increase later in the disease, but a subset exists for whom those symptoms develop early on.
Fox's initial symptoms were a mild tremor, resulting in a motor score below 10.  In most cases, the mental, behavioral, and emotional scores rise later in the illness, but some patients experience these symptoms earlier on.