One can be forgiven in thinking that the mirror is perfect. While things occur similarly, they're not always the same on either side. Oftentimes people, ponies, and even situations themselves are reflected.
Like an older sister who lost her younger, or a younger who lost her elder. Separated by the smallest possible distance and simultaneously the largest, through the mirror and across a dimension. A pair of pins at greatest possible distance, a perfect seven-ten split.

But sometimes you can pick up the spare.