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Parts binning. There aren't a million different CPU made in different for every clock rate, which is often referred to as clock speed, so I'll just call it that. There are a few versions made, then after manufacturing they are bench tested. There are no perfect chips, and the imperfections can cause the chips to be unstable at when clocked at higher speeds. So say, for example, one line produces chips that are designed to be clocked at and certified at 1Ghz. Not all of those produced will actually be stable at that speed, but some can be certified at 800Mhz. At many of those that aren't stable there will be stable and can be certified at 600Mhz.
Now take that 600Mhz chip that couldn't be clocked at 800Mhz. Perhaps it could also be clocked at 786Mhz, but the next step down from the manufacturer is 600Mhz, so it will be certified and sold as a 600Mhz chip. The thing is, a chip that is only stable when clocked at 620Mhz would also be certified at 600Mhz. There is nothing deceptive about this, Qualcomm (or whoever) is selling a 600Mhz chip and both of these would indeed be stable and meet specs at that speed. The only time most people would know the difference is when they overclock the cpu and rediscover what the manufacturer determined during bench testing.