"Fastest" is a term used and misused in many ways. Generally, if a car goes to 60 mph faster than any other car it's the "quickest" and if it has the highest top speed it's the "fastest."
Ferrari calls the Ferrari 599 GTO the fastest street-legal Ferrari ever built, despite having a lower top speed and slower 60 mph time than the Ferrari Enzo. The reasoning? The GTO is faster around the Fiorano test track.