The SPDIF protocol provides locations in each subframe for 24 bits. They might simply (and misleadingly) be basing their statement on that, and setting the 8 least signficant bits to 0, or they might really be using some or all of those bits. There's no way to tell without more information than is provided in the manual (which I took a look at, btw).
Regards,
-- Al
Regards,
-- Al