One way to interpret is to compare to the one dimensional case. Here, the gradient reduces to d/dx, and the vector R is (x-a). Hence, it is clear that the derivative of x-a is simply 1 at all points in the space. In 2 or 3 dimensions, this still works, only the gradient is a vector with more than one component. Still, the change in the position with respect to the position is always 1, even in this vector form. This works relative to any point in space because derivatives of constants (e.g. a, b and c) is zero.