You're last thought is on the mark. Even CDs that are bundled with the DVD-A version of an album were usually from different masters.

The only way to judge is to take a 24-bit wave, and dither it down to 16-bits and compare those two. I've done that, and can't tell the difference. In fact taking the 24-bit wave, encoding it with Lame using the "extreme" preset, and then turning it back into a 16-bit wave, I can't tell the difference from the original.

We don't need better distribution, we need better mastering.
Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs