The present paper addresses the computation of force fields inside permanent magnets that are used in many magnetic systems. The aim is to stress that magnetic behavior is not sufficient when the B-H constitutive law depends on the mechanical deformation. A full knowledge of B and H may not be enough to unambiguously compute forces. Using two expressions of magnetic energy, both admissible and giving the same B-H relationship, an application of the virtual power principle leads to two distinct force distributions. To prove this, thorough computations are performed: depending on the chosen expression huge differences on deformations are obtained.