morphs can be created which only effect a small portion of the bodypart, (I have created jcms which just effect a Dozen verts with MLP),  my guess is that Zbrush is displacing the mesh slightly on you,  or the tools are somehow effecting more of the mesh than you intend, causing the vertex movement you are experiencing on the opposite side. have you tried masking parts of the mesh off in zbrush to see if that helps keep them from being effected?