The "official" name for DDR memory is based on its bandwidth rather than clock speed. The easy method to convert data rate to bandwidth is to multiply by eight. Thus, DDR-400 is called PC-3200; DDR2-800 is called PC2-6400 and DDR3-1600 is called PC2-12800.
The math behind this conversion factor is simple: PC memory modules based on SDRAM technology use a 64-bit connection; there are eight bits in a byte and 64 bits equal eight bytes. For example, DDR2-800 transfers 800 megabits per pathway per second; its 64 pathways provide one eight-byte transfer per cycle and 800 times eight is 6400.
The problem comes with "rounding" and was first noticed with DDR-266 (PC-2100). The data rate of 266 MHz is actually 266.6 (continuously repeating decimal) megahertz, so the true transfer rate was 2133 MHz.
Today's DDR3-1333 has a peak bandwidth of 10666 MHz, which can be improperly rounded down and called PC3-10600, rounded up to be called PC3-10700 or stated without rounding as PC3-10666 depending on the manufacturer's desires.
Buyers will find that searching some venders for multiple DDR3-1333 brands will require them to check all three "ratings" to view modules of the same actual speed, but most brands label their DDR3-1333 products as either PC3-10600 or PC3-10666.