An examination of the top view of the first example (left) and the side view of the second (above) will show why this works.
Note that once the width or height lines are drawn, any set of parallel lines will strike off equally-long rectangles by becoming their diagonals. And since they are parallel these lines naturally converge to the same point in perspective (the diagonals' vanishing point).
 Drawing Equal-Sized But Unequally-Spaced Elements - Vanishing Point Of Diagonals Method
Suppose you had determined one point on a rectangle (such as one of the knobs of a radio) and wished to locate another symmetrically.
(The following procedure applies both to top view and perspective drawing.) 1st: Draw diagonals. 2nd: Carry around guide lines (arrows) as shown. Basically, this creates a concentric rectangle. 3rd: Draw line parallel to side of rectangle (shown dotted) to locate the desired point.
Diagonals As An Aid In Drawing Concentric And Symmetrical Patterns On Rectangles and Squares 
Within a square or rectangle, a multitude of concentric patterns can be drawn in correct perspective by bringing horizontals to their vanishing point, drawing verticals, and "turning" the pattern at the diagonals.
Essentially, the diagonals allow you to "carry the pattern around," thereby maintaining symmetry. Studying the side view of the design at right will help explain how this works in perspective.
Many symmetrical patterns can be drawn accurately and quickly by using diagonals in this manner.
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