This is due to how Slic3r based Slicers including Bambu Studio and Orca Slicer assume the shape of the extruded lines, as well as compensate for the gap between the assumed shapes by overlapping the extrusion lines.
Without any feature such as the experimental “precise wall” functionality in Orca Slicer, two 0.4mm walls at 0.2mm layer height never equal 0.8mm but actually 0.757mm (according to Andrew Ellis) or 0.714mm (according to SoftFever), as lines are put next to each other with an overlap. Otherwise you’d have a gap in between the lines, due to the round edges of the line.
Andrew Ellis has a great explanation of this in his Tuning Guide:
And there’s a great explanation of the Precise Wall feature by SoftFever, as well: