GEoff what you just said is logically consistent but logic alone achieves nothing without facts.
The fact would be that there are differences possible and that any individual may or may not hear it in any particular case. One case means nothing other than two different outcomes with two different people.
Now expand the scope to consider many similar cases, and then at least statistically there is a basis perhaps to draw some conclusion. Still does not prove anything though about any particular case. Only that something expected might be likely or not.
The fact would be that there are differences possible and that any individual may or may not hear it in any particular case. One case means nothing other than two different outcomes with two different people.
Now expand the scope to consider many similar cases, and then at least statistically there is a basis perhaps to draw some conclusion. Still does not prove anything though about any particular case. Only that something expected might be likely or not.