|They are getting the 1600 by upping the voltage. Could also be that the chips on the module are not first quality and require more voltage to even work. |
The JEDEC standard for DDR3 is 1.5v. You could try the advanced memory settings and loosen up the timings first(larger numbers) then try 1866.
Download and run HWMonitor in real time to see what is happening with the voltages. If the voltages stay within +-5% of the rated 3.3v, 5v, & +12v then you may be OK with what you have. Get HWMonitor from the link below.