Yeah memory overclocking on Ryzen is a little complicated, since there are multiple physical chiplets in the architecture (IIRC for 5000 series each physical chip is treated as a discrete unit, unlike previous where each physical chip was treated as 2 discrete units). By default when overclocking the memory up to a certain speed, IIRC 3733, the FCLK (the clocks peed of the "infinity fabric" which allows the memory controller and various dies to communicate) will try to match the speed of the memory, in a 1:1 ratio. This will be displayed as half the DDR memory speed (so 1800MHz FCLK for 3600MHz memory and so on). Once you exceed a certain speed, either 3733 or 3800 depending on BIOS, your board will force it into a 2:1 ratio, in other words for 4000MHz memory your FCLK will only run at 1000MHz, since it cannot manage 2000Mhz. Some BIOS will also show you a UCLK option, but for practical use you almost always want this to match the FCLK.
In Nebulous' case, merely speculating, he was unable to run the memory at 3800MHz 1:1 and by going to 4000Mhz it kicked the FLCK ratio down to 2:1 allowing it to be stable. My system cannot run a FCLK of 1800MHz either, but given the older hardware and sketchy nature of Ryzen BIOS updates, I'm plenty happy with what it's been able to manage.
Finally unless your usage case specifically thrives on memory bandwidth, performance will usually be better with a 1:1 ratio unless you can reach astronomical memory speed.
