While the iDRAC inventory of the Messaging Gateway 8360 and 8380 hardware platforms show the installed memory capable of 1600 MHz, the CPU in some hardware versions is limited to 1333 MHz. This places a 1333 MHz upper limit on operations and the memory will "clock down" to match the CPU speed.
This becomes apparent when comparing the iDRAC hardware inventory with the kernel startup messages at boot up.
This is expected behavior due to a mismatch in CPU and memory speed limits.
This is a known issue with some SMG hardware appliances but does not affect virtual infrastructure installations.