When learning a song by ear, we tend to focus in on the highest part being sung. I think with bluegrass in particular the high tenor part is a harmony part that's confusing your ear. In Sacred Harp & the other old harp books the tenor & treble cross frequently, but in general the melody is lower than the treble harmonies. I think our treble is similar to a high tenor. They can't really be learned from a recording by ear. Some of the original bluegrass singers came from that old book singing background & that may be where it comes from.

To further confuse you, Emmylou & Ricky probably freely modify whatever the basic 'original' melody was to start with. You might want to try listening to other's versions & see how they compare. See if you can hear a sort of consensus tune.