[phenixbb] R/Rfree can not be reproduced

Dear all, I had a weird case in which I can not reproduce the R/Rfree value for the same refinement. "same" means I run my refinement first and then I rerun with "phenix.refine *.eff --overwrite". The R/Rfree values for the two cases are different (0.2942/0.3190 vs 0.2942/0.3085). Should not they be the same always? the command I used: 1st. phenix.refine new-unique.mtz model.pdb xray_data.high_resolution=2.7 fix_rotamers=true strategy=individual_adp+individual_sites+tls ../tls_group_selections.params wxc_scale=0.05 refinement.input.xray_data.labels=FP,SIGFP --overwrite 2nd phenix.refine model_refine_001.eff --overwrite thanks, Wenchang