Re: Asrock G41C-GS and Corsair Platinum XMS3 1333MHz issues.
It's complicated but rest assured the 664.9 is correct and you are running the mem at the prescribed 1333.
664.9x2=1329.8 , some MB/BIOSs hit it spot on while others come very very close to rated. There's nothing to worry over.
For the same reason, CPU-Z shows my 1600Mhz at DRAM Frequency operating at 800Mhz.
DDR = Double Data Rate
Frequency and Memory Rate are two different things.
Mine: ASRock Z77 Extreme6, i7-3770K, 16GB Samsung MV-3V4G3D/US, SeaSonic X-750 Gold, On-die HD 4000, Corsair H80i, Antec Spot Cool, Fractal Design R4 Black Pearl w/Window
Hers: ASRock 990FX Extreme4, FX-8150, 32GB AMD/Patriot AE316G1601U2K, SeaSonic X-750 Gold, Asus HD7770-2GD5, Corsair H80i, Antec Spot Cool, Fractal Design R4 Black Pearl w/Window
Server: Gigabyte GA-990FXA-UD5, FX-4170, 32GB AMD/Patriot AE316G1601U2K, Enermax ELT500AWT, IBM ServeRAID M1015, Sapphire Ultimate HD 4670, Norco RPC-470