[phenixbb] Sulfate ion on 2-fold axis

Hi all, I have a sulfate ion sitting on a 2-fold axis of symmetry. If I set all the atom occupancies to 0.5 and refine using phenix.refine (Phenix v. 1.5-2) the occupancies are changed to 0.00 for the sulfur, 1.00 for three of the oxygen's, and 0.72 for the fourth oxygen. Can anyone explain this? Thanks in advance, Jon