[phenixbb] B-factor discrepancy

I am refining a single-mutant structure at 2.1 A. The native was solved years ago by heavy atom phasing. The reported average B-factors for alpha carbons especially of the original structure were significantly lower than the ones I am seeing. around 15 vs. around 25. Has anyone any idea if this is perfectly fine and its simply something intrinsic to my crystals? Also at this resolution, should I try to refine any residues at all anisotropically? Thanks for the guidance