RAM posts at the programmed SPD settings which is NOT the maximum RAM speed the DIMMs have been certified to run at. If you have a Phenom II CPU the default RAM frequency is 1333 MHz. If you have a Bulldozer based Zambezi the default RAM frequency is 1866 MHz.
You can change the BIOS settings to see if the RAM will run at 1866 MHz. if you have a Phenom II CPU. It may or may not. You may need to adjust the RAM voltage and timings as listed on the RAM label or the package the RAM came in. As noted by werner123 most mobos revert to a default RAM frequency when all slots are filled.
If your CPU is a BD based Zambezi then you should have no issues adjusting the RAM to the default 1866 MHz.
The memory controller in the CPU will determine how fast the RAM is going to run.
"motherboard natively supports ddr3 1866" means exactly what it says. If you plug DDR3-1333 RAM into the motherboard, it is not miraculously going to run at 1866. And if you plug DDR3-1866 RAM into the motherboard with a CPU that cannot handle the faster RAM, it is not still not going to run at 1866.
@ beenthere if the motherboard natively supports ddr3 1866 should it not run at that speed regardless of the ddr3 1333 build in memory controller on the phenom 2?
Nope. The RAM boots at the programmed SPD. Then if you set it to a higher frequency and it runs stable it will boot to that frequency in the future.
When a mobo makers states it's mobo will support RAM frequencies up to 2xxx it means the mobo has been tested and is capable of running RAM at that speed. It does not mean that your CPU or RAM will however run at the higher than CPU/RAM default speed, which for the Phenom II CPUs is 1333 MHz. and for the Bulldozer based Zambezi is 1866 MHz.
The 'Defaults', seem, to be both CPU and JEDEC driven. Clearly if you place a pre-Zambezi CPU in a '990FXA' then the Default is DDR3-1333 'SPD', and in contrast if you place a Zambezi e.g. FX-8150 in the same '990FXA' MOBO you get the highest 'SPD' (JEDEC standard) up to DDR3-1866.
However, most DDR3-1866 are also XMP which is an Intel standard only so if the 'SPD' is JEDEC 800MHz and the next is XMP-1866 the Default will be DDR3-1600.
So to run DDR3-1866 in this example you'll need to set the Memory Frequency or Memory Multiplier manually to get the Zambezi's native DDR3-1866 Frequency.
I said "seem" because there's little data that 'I've' seen to state it any other way. IF I see something different then I'll edit/update. I do not have an Zambezi CPU, I'm all Intel now.