This is what Rolex has to say about the term "Superlative Chronograph Officially Certified."
SUPERLATIVE CHRONOMETER
The four simple words SUPERLATIVE CHRONOMETER OFFICIALLY CERTIFIED engraved on the timepiece’s dial mean that the movement of this watch has endured 15 days and nights of testing by the COSC (Contrôle Officiel Suisse des Chronomètres), an independent not-for-profit association.
To receive COSC certification- a watch must demonstrate
extreme precision in a variety of positions and temperatures– the standard level of quality for every Rolex Chronometer.
http://www.rolex.com/sites/4/e-brochures/explorer.pdf
That may not explain everything, but that's what Rolex has to say about it.