I just came up with a question that I've never seen asked or talked about. It maybe obvious to everyone else in the world but I realized I simply can't answer it except with the non-answer.

This is the question:

***What distinguishes a melodic line, such as a vocal line from being sung out of time vs being "in time" yet off the beat?***

This is probably not clear so an example will do:

We know that there are many melodies that anticipate the chord by a fraction of a beat such as a 16th or even a 32nd note. Any score of the masters will have this happening. We don't consider these notes "off" or "wrong" or bad timing. Now suppose a singer is singing a line and "feels" some words are coming in sooner than one would normally expect from analysis(e.g., strong syllables in strong positions).

When I do this, sometimes it feels like my words are anticipating the beat, like the some melodic lines do and almost feels like a timing issue(but I'm not sing out of time, I do it repeatedly). Something like a vocal line with rhythm of a 16th's followed by two 8th notes when the accompaniment is in eights.

Some people may say that the lyric would be sung wrong because it is not landing in a strong way and may feel like a "double hit". (as if anticipating the beat accidentally by a 16th).

I can imagine many cases where this is done more or less. The question is, why does our brain allow some patterns that are "off" to sound right even when things don't line up(or maybe it's just wrong). Is it mainly due to repetition or is there some fundamental law about making the syllables line up in strong positions(which are created by the previous patterns, which kinda goes back to repetition), or something else?

I'm just curious is there is a sort of mathematical way that one can verify if a setting of a lyric is "good". I know there are "books" that people claim things, but those are not proof but just methods that people have used in the past. It seems we want the lyrics to be most intelligible, which instruments don't have that problem, and this creates a set of constraints(e.g., an easy line has to be loud enough to be heard).

*So, alternatively, does anyone have any idea how to show(semi-scientific way taking into account acoustics, psychology, physiology, etc) convincingly why one lyric setting works with one accompaniment and one doesn't(I'm not talking about specific cases but general principles)?*

(e.g., children's music should have simple more straight rhythms because it is easier for the child's brain to process simple regular patterns)

Or do we go with the non-answer and just say "If it sounds good!"?