Ductile bulk metallic glass of composition 53.0Zr-18.7Cu-12.0Ni-16.3Al (at%) is plastically deformed under uniaxial compression and observed in situ by synchrotron high-energy X-ray diffraction. The diffraction patterns reveal the induced atomic strain is orientation dependent. At the onset of plastic deformation, the atomic strain in the compression direction saturates to a close-nearest-neighbor distance while atoms relax in the transverse direction. The ever increasing transverse atomic strain expresses in an augmentation of the apparent Poisson's ratio up to ν = 0.5, which is consistent with volume conservation. Contradicting phenomena from linear mechanics, such as the non-vanishing shear modulus at ν = 0.5 can be explained by the non-affine character of the deformation, giving rise to characteristics of a localized martensitic phase transformation. The findings explain the often-reported phenomena such as, the high Poisson's ratio values found in metallic glasses, the partially liquid character of the structure, the free volume increase and the Bauschinger effect.