When the flare nut is screwed all the way in, the tubing flare ALMOST seats all the way, but not quite. It is still not clamped against the inverted flare well enough. Even with the nut all the way tight, the tubing will still move, albeit with some binding. I'll get a video of me tightening a piece of flare tubing/nut that I have lying around (cut from some fubar'ed lines) and I'll demonstrate what I mean.