Maybe ANBU is just a part time job. Ninja's that are good enough for the job gets picked by the hokage or something. When there are no missions on then they just go back to the regular Ninja rank and role. I kind of thought about this when Raidou (ANBU Captain we saw when Third-Hokage was attacked by Orochimaru) got owned by the sound 4 while with the toothpick Jounin. At that time Raidou was actually wearing a normal Ninja uniform with the vest and shit. So yeah Kakashi could've been both ANBU and Jounin at the same time until he quit.