CPU Speed Difference

"My Computer->Properties" at work says:

Intel(R)
Pentium(R) D CPU 3.60GHz
3.59 GHz, 2.00 GB of RAM
Physical Address Extension

That looks Ok to me. My home computer says:

Dell Optiplex 755
Intel(R) Core(TM)2 Duo CPU
E6750 @ 2.66GHz
1.97 GHz, 1.96 GB RAM
Physical Address Extension

Why the difference in rated vs actual processor speed on the home computer?

Thanks,
- Arnie