Because the symmetry or asymmetry of a figure or object is determined by how it is decided on each side of a line of symmetry, that references the balance on either side of the imaginary line of symmetry.
Proportion relates to the size of a form, not the symmetry. Emphasis has to do with how a piece is viewed and what was done to the surface to create highs and lows in the work. And repetition is a motif that gives a feel of movement to a piece of artwork.
