Different CPU info on Windows and Linux?

Even without overclocking or underclocking, modern CPUs change their speed, it can enter turbo mode or it can enter power saving mode (and dip way way down). The difference in the exact background processes running is what accounts for the difference in reported speed.

In fact, for many CPU monitoring programs you can observe the changes in speed in real time as you run / close programs

Would recommend something like Speccy to get accurate specifications of your PC.

Thing is Task Manger on Windows 8/8.1 always show the present clock. Sometimes when some type of power saving is enabled(especially in Laptops) the processor "Underclocks" on the go to save power and you will see a smaller number in Task Man.