Dunno about Robbies specifically, but for Bell:
the underslung design of the head puts the rotor’s pivot point more into line with the blade’s normal C of G when in flight, and makes it more stable:
If it were otherwise, when the blade flapped up, the C of G would be more inboard:
In this way, most of the bending stresses in the lead-lag plane caused by coriolis effect and geometric imbalance when flapping are relieved (that’s from Bell - the exam answer is: preconing relieves bending loads in the plane of rotation). Preconing is the setting of a blade angle to a slight incline when the helicopter is level - on the Bell 206, it is 2½°. The centre of mass stays about the same distance from the mast after the rotor is tilted.
Phil