what is that 1% case where restoring force is not equal to deforming force?

body is said to be elastic if it suffers a deformation when a stretching or compressing force is applied to it and returns to its original shape when the force is removed. For example, suitable forces can stretch a coil spring or a rubber band and they can bend a flexible rod or a beam of metal or wood. Even bodies normally regarded as rigid, such as the balls of a ball bearing made of hardened steel, are somewhat elastic -- they will deform if a sufficiently large force is applied to them.

The force with which a body resists deformation is called its restoring force. If we stretch a spring by pulling with our hand, we can feel the restoring force opposing our pull. The restoring force and the force that produces the deformation are of equal magnitudes; they are an action-reaction pair.

Under static conditions, the restoring force with which an elastic body opposes whatever pulls on it often obeys a simple empirical law known as Hooke''s Law:

The magnitude of the restoring force is directly proportional to the deformation.

This is not a general law of physics -- the exact restoring force produced by the deformation of an elastic body depends in a complicated way on the shape of the body and on the detailed properties of the material of the body. Hooke''s Law is only an approximate, phenomenological description of the restoring force. However, it is often a quite good approximation, provided that the deformation is small.

The deforming force and the restoring are acting on different objects. The deforming force is the external force applied to the object, lets say by your hand. The restoring force is the force exerted on your hand by the object due to Newton’s Third law.

The two forces are equal and opposite but since they are acting on different objects, they do not cancel each other out.