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.
The initial symptom of Fox was a mild tremor, resulting in a low motor score.  In general, mental, behavioral, and emotional scores rise later in the illness, but some patients experience these symptoms earlier on.