Why are different mixes on a surround sound disc not released at similar loudness?

[Dynamic Editor]

One thing that I've noticed lately when listening to sets with multiple mixes, such as the recent Rhino Atmos sets, is that often the songs aren't made similar loudness between different mixes.

Here's an example. These are the mixes of "Second Hand News"—Atmos, 5.1, and stereo respectively—found on the recent Fleetwood Mac Rumours Blu Ray, each folded down to mono:

The reason I fold it down to mono is so it's clear how different the overall loudness being presented to you is. The Atmos mix is about 3dB quieter than the stereo mix, and the stereo mix is about 3dB quieter than the 5.1 mix.

Listening and comparing the mixes is frustrating, because whenever I want to compare the mixes by swapping to a different mix, I have to fiddle with the volume on my receiver. I have to ask: Why do companies not lower the loudness of the final masters so each mix retains a similar loudness?

I'm not asking for a remaster of the mixes in question, to be perfectly clear. I also understand the loudness requirements of Atmos mixes on streaming services basically require those mixes to be quieter. But I can't understand why already-mastered 5.1 and stereo mixes don't just have their loudness lowered before pressing them to the disc. If anyone has technical or industry knowledge on why that is, I'd definitely like to know!

 

[edisonbaggins]

Interesting topic. Check out the Joni Mitchell box, if you have it.
I'm actually finding the 3 mixes on each disc to be fairly well level-matched.
It is very pleasant to not adjust my volume by 12db.
I find myself nudging the volume a click or two or three, but I chalk that up to preference.
I really think they tried level matching her vocal. I focus more on drums, if present.