Memory is "binned", like hard drives are binned. What is binning? A factory line sits and cranks out hard drives, thousands a day. The goal is to get, say for instance, 1TB of storage out of each drive off a particular line. So the drives come of the mfg line, go into final certification and viola! - some of them have bad spots on the discs. Those bad sectors are a dinger to the total capacity, so much so (sometimes) that huge areas of the disk are unusable. But is the drive a complete bust because of it? Nope. They simply de-rate the drives storage capacity to something lower and sell it as a smaller drive. The process is routinely referred to as "water fall". Some drives come out with the full 1 TB. Others only yield 750 GB, and so on... It's the same for memory. The dies that make up the base memory space, multiplied by the number of dies (chips) per DIMM stick are designed for a maximum speed with best case latencies for a specific voltage. Some don't make it to the top of the heap and therefore get de-rated. It all comes down to what they are certified or guaranteed to run at. Hence, same mfg, same size, same voltage - different speeds because of effective (tested) yielded results. And it gets more complicated with silicon too. Higher speeds require higher voltages on some chips. Other designs can get away with blazing high speeds with lower voltages. It's all about the trade off between design types and yielded manufacturing results. Lastly, some memory is "green" thereby being low voltage and subsequently lower power to help keep a build at power sipping levels. Enthusiast memory is typically balls out fast, at say 1.5v, but designed with margin to go faster with 1.65v - 1.70v as long as you go into the BIOS and massage specific areas of the timing table.
If you're not a gamer/enthusiast, stick with the recommended memory configs published by the mfg of your motherboard. And stick with reputable brands. You can't go wrong with Crucial btw!
Hope this helps...