Expanding compound path causes a shift in the object shape
If I call Expand on a compound path, there is a shift in the width and height of the object. This seems to only be a problem when the stroke is in the center. If the stroke is inside or outside, there isn't a problem.
Illustrator can display width and height values respecting the width of strokes and effects, if you enable Use Preview Bounds option in Preferences > General. It also makes bounding box larger to fit them visually, hence the change in dimensions.
When it’s off, only geometric shape is taken into account, ignoring stroke width or effects.
When you expand the shape, the stroke is not a stroke anymore, but a filled path, that makes the overall group geometrically larger.