Fyi to OP and anyone else trying to get a handle on changes in western music through history - I became very interested in this topic and scoured the internet (i.e. published academic research available on the internet) for information -
All of these recordings NEED to be understood as 'creative reinterpretation' at best, regardless of what the liner notes may imply... there are HUGE largely-unanswerable questions about fundamental aspects of performance - singers' tone production (this means, the difference between opera voice, European folk voice, 'middle eastern singing' voice, Chinese opera voice, etc. - these norms are subjective and change dramatically over time), ornamentation (adding all kinds of extra notes, sliding between notes, etc.), use of instruments, tempo, rhythmic emphasis, etc...
Even today's performance style for Mozart is drastically different from the performance style that you hear on really old recordings from like 1910-20, so imagine hundreds more years of changing tastes (and without the impact of widely-available recordings keeping everybody more on the same page!).
Western notation started as a memory aid to help preserve a more complex oral tradition, nobody would've been expected to learn how to sing a piece just from looking at the notes.
Also, if you know the famous history of polyphony evolving out of monophony at Notre-Dame - it's actually becoming clear now that there was already an established tradition of improvised polyphony, which the Notre-Dame composers drew on. That's just one example of a little detail that suddenly upends the established image of the past on which many of these recordings are built.
Suffice to say it is MORE THAN LIKELY that the music of Perotinus or Leoninus etc. sounded, in its day, almost nothing like you hear on record, and, as a related point, it is an illusion that western music 'started simple and became more complex over time' - what IS true is that NOTATION became more complex and more important over time, and complexity was redistributed into forms visible in the notation (i.e. structural/'architectural' complexity gradually became more prominent)...
I would be happy to provide recommendations of the relatively few records which actually DO try to explore genuine variety of approaches to performance.
EDIT: to avoid getting into 'well, what is complexity, how do you measure it' - what I really mean is, it is an illusion that western music NECESSARILY 'started simple and became more complex over time' just because of increases in notational complexity - notation is not evidence enough. Also want to acknowledge that obviously 'almost nothing like you hear on record' is totally subjective, I don't want anyone saying 'well they got the notes right' (even there we have questions - look up 'musica ficta' - even into the 1500s it was standard for performers to add sharps and flats to the score DURING performance!)