They do it in the digital domane, so you reduce the resolution. If you reduce you volume by 6 dB you will divide the resolution by 2. It is okay to do this with a 24bit signal when you can live with this :
Well .. if you use a 24Bit dac and feed it with the 16 bit signal as 24bit where you don't use bit 0 - 7. Then the digital filter will "convert" this signal into a 24 bit signal.
Why? The dac's filter do 24bit calculations.
A bit hard to understand... But you will only have big sound change under -48dB.