It's kinda like engines. You can build a 1.6 Litre engine which will give a certain amount of power.
You want to produce 2 cars, a 'boring' one, and a 'sports' one. So rather than produce a whole new engine, you can simply tune the hell out of the same engine to increase the power output.
This is what the chip manufacturers do. They produce a new CPU core, but they need to sell to many different market sectors. So they 'down-bin' some of the cores to sell as cheaper, slower chips, and sell some as faster, more expensive ones. These settings are just minor modifications to some of the circuitry on the chip, but don't have to affect the core itself.
Often, at the start of a core's life, they'll test them, and due to [very minor] differences between each individual core, some will be able to run faster than others, so they'll use the 'failed' cores to sell as the slower speed ones. But usually after they've refined parts of the process, most of the cores are very close in terms of quality, so they have to sell some cores as far slower than they're capable of. The 'Barton' core Athlons are like this, as are the Pentium 4 'C' series (and others of course).
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<font color=red>The preceding text is assembled from information stored in an unreliable organic storage medium. As such it may be innacurate, incomplete, or completely wrong</font color=red> :wink: