Tungsten, W - m91 file question

Hello,

I am writing to you about tungsten file m-91 (https://materialsproject.org/materials/mp-91/).
In there, and the other W data available, non of the unit-cells seem to depict bcc structure, but rather cubic structure at most, and the lattice parameters are not close to known tungsten lattice parameter from XRD or other sources.

Why is there a variation like this?

mp-91 is the BCC structure, the unit cell and lattice data shown is in the primitive cell setting. Conversion of the mp-91 primitive cell to the conventional cell yields a lattice constant of 3.187, which is pretty close to the ~3.16 from experiment.

>>> from pymatgen import MPRester
>>> mpr = MPRester()
>>> struct = mpr.get_structure_by_material_id("mp-91", conventional_unit_cell=True)
>>> print(struct)
Full Formula (W2)
Reduced Formula: W
abc   :   3.187414   3.187414   3.187414
angles:  90.000000  90.000000  90.000000
Sites (2)
  #  SP      a    b    c
---  ----  ---  ---  ---
  0  W     0    0    0
  1  W     0.5  0.5  0.5