X370 Gaming K4 BIOS 3.30 9/14/2017

If you look at CPUz under the memory tab you will see JEDEC and XMP columns. XMP (eXtreme Memory Profile) is a little different as it forces the RAM to operate beyond it's components rated speeds, typically with higher than rated voltages. 

In other words, this is a good thing, it means that AM4 platforms on that BIOS will default to DDR4 2666 even with XMP turned OFF in the UEFI, provided the RAM has a JEDEC 2666 profile. You would need to make sure the kit supports JEDEC 2666 at the time of purchase however. Most currently available kits only support 2133 or 2400 via JEDEC. 

All in all it isn't all that important to the average consumer, most users will use XMP and overclock their RAM as high as they can get it. For the enterprise/workstation user however JEDEC is a much more robust and stable standard and is used preferentially over XMP in data centers, workstations and the like. At the end of the day, gaming and daily use require a lot less stability than something like a high end render station that can peg all available threads and memory at 100% usage for hours on end.