I'm not an electronics engineer, so don't crucify me, Ok? Lol
But, from what I understand, CDs are PCM data, pulse code modulation. So the PCM datastream is not only digital steps, but also sampled at uniform intervals. Therefore the data must also include a clock somewhere. Even if the data itself is identical, the sampling timing is also critical.
I suspect, but can not prove, that its the clock or the timing that's responsible for the difference in sound between different units. I have three transports here, a PS Audio Perfectwave, a PS Audio Directstream, and an Esoteric K-03X, which can be used as a transport. The three units all sound different when feeding the exact same rest of the system. Which led me to wonder if the NAD might sound differently than, say, an Aurender playing the exact same rips or data.
PS Audio, on their website, states the betterment of sound from the Directstream over the Perfectwave is a result of better data timing, due to an improved memory buffer, and an ultra-low jitter fixed clock.
I also suspect its that timing aspect that results in different sound from different digital interconnects. Some are better at not messing up the timing than others.