Found some info on Segment Statistics in this post: https://discourse.slicer.org/t/segment-statistics/7303
My interpretation is one of the volumes is computed using a closed surface representation of the segment (similar to what you see when you press the 3d button in segment editor with surface smoothing ticked), and the other is computed by adding up the volume of all the individual voxels making up the segment.
I am guessing Volume (1) is the closed surface representation in this case? Volume (1) is slightly less in volume and agrees closely with the Blender computation which is based on surface models.