I have always assumed that the phenomenon Richard points out is a stylistic thing. The singers choose to sing that way for their own reasons.

Suzanne Vega was the first practitioner of this style to attract my attention.

Why do they want to sound like that?

Why did we all try to sound like Dylan once upon a time?

Some of us are sounding like our parents in this discussion.

Russ (Permanent GUEST and late to the party as usual)