I admit that the title of the question sounds a little strange, but I wanted to posit a more general question based on a specific song I was looked at.
I was looking at this arrangement of Chega de Saudade for solo voice and guitar, from a recording from the 2000 album "João: Voz e Violão" (youtube, spotify)
In this minimalist arrangement, João Gilberto makes great use of 3 note voicings, which can serve different (and sometimes ambiguous) harmonic functions.
This voicing, for example:
X: 1 L: 1/4 K: clef=treble-8 M: none [A,^F c]
can serve variously as Am6, Ao7, or D7/A depending on the harmonic context.
Now, there's a passage that has the following melody (syncopation omitted for simplicity's sake):
X: 1 L: 1/4 K: Cm clef=treble-8 |"Cm7"E2 G,2|"Eb7" C3 E |"D7" D2 =A,2|C3 D |"Abm6" E2 _A,2| =B,3E|(DC)|
And I've written the chords (as I think it's fair to say they're implied by their 3 note voicings) on top.
This all seems like a nice harmonisation; a particularly interesting moment is the second bar; conventionally you would simply hold the C minor chord but in this arrangement the guitar plays a rootless Eb7/Bb in the above voicing. Along with the vocal line you get a nice Eb13 chord (missing the root).
Except... the vocal line is actually low enough that the "13" of the melody is lower than the "7" of the chord, so it's really a 6. As expected, if you actually play the notes voiced in this order on one instrument, it sounds awful, yet when sung against the guitar line it sounds wonderful.
Now, at first I thought I could chalk this up to just a "separation" of the voices; because the guitar and vocal have such different timbres and both have good voice leading (the guitar part at this point simply steps down chromatically), the clash works in a way it wouldn't if voiced on the same instrument.
And that would make sense, except, if that was the case, shouldn't I be able to reverse the voicing (putting the 7 in the vocal and the 13 in the guitar) and it sound alright (if not better)?
As it happens, it's relatively easy to adapt the melody so that it lands on the Db not the C. And when played against the 7th or a 9th chord, as expected, there are no clashes and it sounds fine (if less interesting). But when the chord is changed from to a 6 or 6/9 chord, it sounds terrible. However, the original melody sounds fine against the 6 or 6/9 chord. Now, a voicing of an Eb13 chord should always have the 7 below the 13 to sound good (it's just one of those rules, like a minor third over a major sounds fine in blues but never vice versa). And when playing these note clusters (including the note sung by the voice) on one instrument, the voicing with the C above the Db sounds much better. So why should it be different for when that note is sung?
My only explanation is that, because we are hearing a vocal line against a guitar line, even though the pitch of the voice places it in the middle of the guitar chord, we perceive it as above it for some reason; it's the only way I can make sense of which melody/harmony combinations work and which don't.
Having read this question over, it being quite wordy, here is a sketch that shows what I mean more concisely: