Voltage differences between different S codes for same speed chip.

I noticed that two different S codes for the same speed / same stepping of chip have different values on their specs. Does anyone have any info on why this is and does it affect the ability to overclock some of the chips?

For example:

sSpec Number SL6WU
Processor Frequency 3 GHz
CPUID String 0F29h
Package Type 478 pin PPGA
Core Voltage 1.25 V-1.525 V
Bus Speed 800 MHz
Core Stepping D1
Thermal Guideline 81.9W
Thermal Spec 70°C
L2 Cache Size 512 KB
L2 Cache Speed 3 GHz
Manufacturing Technology 0.13 micron
Bus/Core Ratio 15

and...

sSpec Number SL78Z
Processor Frequency 3 GHz
CPUID String 0F29
Package Type 478 pin PPGA
Core Voltage 1.25V-1.400V
Bus Speed 800 MHz
Thermal Guideline 81.9W
Thermal Spec 70°C
Core Stepping D1
L2 Cache Size 512 KB
L2 Cache Speed 3 GHz
Manufacturing Technology 0.13 micron
Bus/Core Ratio 15