Here's a sweeping generalisation regarding male singers but I think there is some of truth in it. Folk singers tend to sing slightly flat (Leonard Cohen, Dubliners) and country singers slightly sharp (George Jones, Dwight Yoakam).

OK, I can think of one exception straight away. Johnny Cash. Maybe that's why he has more "street cred" among folkies (in the UK anyway).

Feel free to tear my hypothesis to shreds (as if you needed an invitation).