>> is there anything that can be done about that?
you cannot record with 18-bit accuracy in most audio applications -- so, I don't see any reason trying to achieve that
anyway, the DSP recording functionality is limited by 16 bit regardless of the codec resolution
moreover, the codec itself isn't a perfect device, so, the two last bits are probably digital noise...
internal DSP processing depends on the actual effects you are using
in general, it is 24/32/64-bit
/E