This can be caused by CPU stepping speeds to conserve power.
It should still perform fine.
Go to Control Panel --> Power Options, and set the power scheme to always on, this should force the CPU to run at full speed.
If the CPU is still shown as an incorrect speed, contact the seller immediately.
Oh, and check the motherboard manual, there may be jumper settings you need to configure or settings in the BIOS, that could also affect the speed.
I think it's most likely to be stepping though.
Hope this helps.