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.
Example:
CPU:
Memory:
SPD (note the JEDEC 800MHz and XMP-1866):