I think it's down to the way it's worded
I can see what you're getting at, viz, trained session musicians would have learnt all the standard "rules" of music and therefore that kind of thing wouldn't have come to them naturally.

But Blousey's got a point - I do remember one of those "backing singers" from The Human League being interviewed about her vocal on their one hit, where she manages to come out with "the best singers don't always have the great voices." Seriously. Without a hint of irony.
(LongJohnBaldry, Wed 13 Jan 2010, 11:13, 1 reply, 16 years ago)

Great vocalists are not always great singers....
...would be a truer statement.

And for that matter great singers are not always great vocalists.

(My paradox glands hurt.)
(Tugnut Ex of this parish, Wed 13 Jan 2010, 11:56, Reply)

It would.
John Lee Hooker is testament to that. Amazing voice, but would be advised against trying anything technically challenging.

Tom Waits is a more contentious one: very capable singer who chose to rough up his voice and sing like a drunkard, I'd say, but many might disagree.
(LongJohnBaldry, Wed 13 Jan 2010, 12:17, Reply)