For VASP.4.X behavior please refer to section 8.6.
Mind: the calculation of the dipole requires a definition of the center of the cell, and results might differ for different positions. You should use this option only for surfaces and isolated molecules. In this case use the center of mass for the position (for surface only the component normal to the surface is meaningful).
The main problem is that the definition of the dipole 'destroys' the translational symmetry, i.e. the dipole is defined as
Now this makes only sense if drops to zero at some distance from . If this is not the case, than the values are extremely sensible with respect to changes in .