It is where the system bus does not run at 100MHz. For example it runs at 99.8MHz. When multiplied out to get your CPU frequency it becomes more exaggerated. The CPU normally might run at 3.0GHz however with this problem would be running 2.994Ghz. This gets worse when you raise the multiplier to say 40.
This is not only possible on MSI's X370 Gaming Pro Carbon, I have seen this happen to other boards for years now. It is more of an annoyance than anything. Stock benchmarks will show that the board with sagging system bus is a smidge slower. It shouldn't effect the ability to overclock however it will magnify the higher your multiplier is set.