Your comment is what every textbook says and it's not wrong for ideal conditions. Meanwhile in the field you come across a boogered up fitting line that leaks,

Your comment is what every textbook says and it's not wrong for ideal conditions. Meanwhile in the field you come across a boogered up fitting line that leaks, you slap some tape on the threads, the nut seals to the ferrule, the ferrule seals to the tube and the tape seals the threads. It's not "right" but it works well enough often enough that people do it.