O.C, next to the RAM frequency specifications, tells us that the RAM isn't automatically supported at that speed, but it is supported and you must manually overclock the RAM it to get those speeds. Basically, you can buy a 2133MHz memory kit, but you need to set it to that frequency yourself in the BIOS, or else it will default to a lower frequency.
Basically, the board supports overclocking your RAM to those speeds, if you want to. Buying an 1866MHz kit and putting it in will not make a difference, it will still be 1866MHz (unless you overclock it further). You don't need to buy faster RAM, it's just telling you that it can support those speeds, but it doesn't automatically recognize RAM at those speeds.
Possibly this web article might clarify things for you.