It could be 'just' temperament vs equal. It's hard to say without hearing the recordings, and I'm not sure I'd know if I did hear it.

Sometimes when I tune my fiddle with an electronic tuner, it still doesn't sound right, and I may be hearing in just temperament. I may just be wrong, too.

I've tried to record a (voice) harmony track using a click track, without listening to the tune. When I play both melody and harmony back together, it sounds off. If I don't play the melody back at the same time, it sounds fine. I figure I'm adjusting to a just tempered scale when I hear what I'm singing along with the melody, and the equal temperament I sing without the melody is just a bit off.

It may just be that the people you named are off the pitch. It's quite easy to do, and I've hear plenty of 'good' fiddle players who seem to have at least a little bit of a problem. You'd think they could hear it when they listened to the recording, but maybe not. Even if they DO hear the oopsies, maybe it's one of those "they won't notice" things.