The technical term is down binning. As I think you know, each CPU comes that comes off the line is test for it's potential speed. Each CPU has some small errors in it's lithography. It is those errors that limit the speed the of the processor (technically, spots of higher resistance cause heat. The more you try to pump through that spot the higher the heat. Too much heat and you get failure). As a CPU matures the number of errors go down and percentage of CPUs coming off the line that qualify for the highest speed increases. The issue is that a lot of people like to get the middle of the road CPU (hey, they are cheaper). So, even though the CPU can perform much better than what is stamped on the cover, the company sells it as a slower processor.
Now, with the Ivy Bridge E, it gets even more complicated because the CPU comes in 2 flavors ... 4 core and 6 core. On the wafer these two flavors are exactly the same, but sometimes you have errors concentrated in one core, so you disable that core (and one more) and call it a 4820. All cores are good, then you call it a 4930 or 4960. Sales of 4820s are brisk and lots of CPUs are binning higher, then you take a beautiful 4960, disable 2 cores and stamp 4820 on it.