The RDC machine tests stiffness by securing the tip and handle and bending the frame in the middle, near the throat/bottom of the head at 6.
So it's only a very broad guide at best like so many other measurements in tennis (eg swingweight is measured as if you're swinging the frame with your wrist).
Other key features which determine how a frame feels in terms of "stiffness" include materials used for construction, thickness at various key points, distribution of weight, head size, and string pattern. All will influence a frame's comfort and perceived stiffness.
A good example is my current frame, the Pure Storm GT. On paper it is at the low-middle end of stiffness around 63. But as others have reported I found it to have some vibration in stock form and play sort of stiff or at least uncomfortable, especially on impacts towards 12. After adding weight to the head and under the grip raising static weight about a full ounce it really does play soft and plush.
By the same token the Pro Staff 6.1 95 is pretty soft on paper but some aspect of the Ampli-Feel technology was very uncomfortable to me. It's designed to increase the feel of the impact which it certainly does!
The softest frame I've used is the Speed 300. It has a low stiffness rating, it is thin beamed, has a large 100" head, and an open string pattern. All of these features provide for a very soft feel but no single feature, including the stiffness value, can be taken to mean a frame is soft or not.