When the revised VMCG was measured there was a distinct effect at light weight. The VMCG value for the aircraft is only 81kts at heavy weights so is probably at the lower limit of what is possible anyway. At heavy weights it was considered that the tyre footprint was greater, providing more resistance to deviation than at light weight as Mad (flt) scientist mentioned.
Another factor in measuring these things is that at light weights the acceleration was so rapid that getting consistent measurements was difficult, especially if there was the slightest crosswind (and there almost always is).
The answer to Panama Jack's first question seems to be "yes but..."
JT