It is not only the spectrum that is different. Libraries treat their stock ridiculously, schockingly bad if you think their bread and butter is their, well, music library.
I am sure that any LMT user has equal or more understanding of extracting the sound information of a medium than the employees of those libraries.
We had recently two incidents that show that very clearly:
* Poorly ripped CDs with skips and outright errors that are clearly visible in the waveform such as the SONOTON albums.
* Poorly recorded LPs. Unfortunately I forgot the person who recorded the SELECTED SOUND vinyls but it sounds so fantastic, instruments super clear (such as big toms in "BIG CITY HEAT") or the outright pristine sound of "NIGHTCAT" makes me really positively shocked on how great LPs can sound. I always have prejudices against that format and there are many caveats that digital/CDs don't have, but it shows that those records are sure delivering incredibly good sound performance. If I remember the original WAV material from SONOTON which is clearly several steps below..
Maybe they even used software declicker/decrackler or EQs while recording... Uff.
Other incidents are:
* FOCUS MUSIC has chirps in their material, very subtle but you can hear the error correction trying to restore the audio. NICHION has the FOCUS music in perfect condition.
* KPM has stained, dirty inlays as their cover art that they even put up on Spotify and their distributors.
This makes me think that SONOTON and KPM have either only one copy left over and if that is damaged, it's over. Or they don't care to go into their vaults and get the masters. Either case is embarrasing.
Now about the different sound quality, please keep in mind that there are so many different interfaces and settings that can change the outcome.
CDs can be ripped with fast settings and they can bring lots of errors or chirps with them - even from a flawless disk. A few "bad sectors" can be error-corrected by stock players even in the first form of the compact disk "rulebook" (there are different "Books", e.g. "Red Book Standard" is the default CD without any perks). These can be inaudible - or not. If you rip a CD in the "fast" setting, you get small errors, it's kind of collateral damage. If I remember our old PC with its 40x speed CD drive, there were lots of bad sectors afterwards that even the burning programs warned about the questionable file integrity. You'd think the software is working, but it bites often more off than it can chew..
If you do it right, you can set up your recorder/ripper to be very careful. RETROMATIC for example does this, he has logs with every rip he does that show 100% perfect and pristine rips. I assume these libraries have their stock of 100+ disks and want them ripped. So they put a guy in an office and he picks the "fine most of the time" settings for speed reasons. There you go: A chirp here, an odd click there.
Then there is the WAV format which can be different, too. PCM audio (what you usually expect in a WAV file) can be modified a lot. Sometimes, older games had PCM music but are actually ADPCM which tries to predict the needs to save bandwidth and this can be faulty too. Sim City 3000 IIRC uses that format.
Then there is a good idea of ChunYinZi that there is tampering with frequencies and sample rates and whatnot. Not sure how much that "resampling" and "dithering" and all that have an effect, some effects may be practically reversible, some might leave inaudible but visible (in a spectrograph) differences.
I am just throwing some random ideas here that showcases all the little things where changes of the material can happen. But now imagine a combination of the above. And each modification is affecting each other until the point you can see - albeit not necessarily hear - it.