With the Intel® turbo boost on the Intel Core™ i5-3570K whenever you are running something you are going to see some boost from the turbo boost. It is set up so that grades down based on temperature and the number of cores in use.
The top speed will be 3.8GHz with only a single threaded app running. Then you would see 3.7GHz with two cores in use and 3.6GHz with 3 and finally you will see 3.5GHz when all 4 cores are in use and you are not overheating. Here is a nice little tool that you can run that will show you as the turbo boost kicks in and does its work. http://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=19105