These models feature an unlocked clock multiplier.
Core 2 Extreme processors are enthusiast versions of Core 2 Duo and Core 2 Quad processors, usually with a higher clock frequency and an unlocked clock multiplier, which makes them especially attractive for overclocking. This is similar to earlier Pentium processors labeled as Extreme Edition. Core 2 Extreme processors were released at a much higher price than their regular version, often $999 or more.
Source[ Wikipedia ]