There is nothing wrong with your system or the memory- like most motherboards, I suspect yours is simply limited as to what multiplier can be used to determine the memory clock. Memory frequency is a result of taking the CPU base frequency and then factoring in the multiplier setting that is specified via the system BIOS. So, if the base clock (or FSB, or Bus in old lingo) is 200, and the multiplier max for the memory is 8x on your motherboard, you can't just select the full 1866mhz.
Most 990-series boards have advanced enough BIOS features to allow you to adjust the base clock and reach the intended memory speed that way. In this example, it'd require ticking up the 233mhz from the default 200mhz. You could then lower the multiplier setting for the CPU to keep close to the same settings there. Again, to make an example, if you had been running the CPU at 4500mhz using a 22.5x multiplier, you'd scale back to 19.5x and be at 4543mhz with the new base clock. Hopefully that came out as making some degree of sense.
The key problem is that there is nobody in the business of selling memory who actually has an interest in explaining what the specifications of their product mean. Memory doesn't dictate the speed it runs at- that's up to the system it's installed into. The numbers promised on the RAM package aren't what they guarentee the memory will run at, but rather what it CAN be counted on being able to run at if you attempt to do so.
I would actually consider running at 1600mhz with tighter latency settings and seeing how that works out as well. 1600mhz at 9-9-9-24 may well be better for your rig than 9-10-9-28. That's a matter of trial and error to establish though. The main thing is to stop overvolting the memory right now though; although slight, there is some risk to running at 1.55v when it's not called for.