Here you have adequately stated yourself that there is bullet obturation. Obturation is more a condition of the bore, than the bullet. The bullet expands to completely fill the bore, and the bore becomes obturated. As a result of this obturation, no gas leaks around the bullet, so leading doesn't happen unless the bullet is of sufficient hardness that the pressure will not deform the bullet enough to cause complete obturation of the barrel, so propellent gasses can escape around the bullet melting a microscopic layer of lead only to leave it in the bore.
Bullet obturation is a myth, however, obturation of the bore by the bullet, as a result of expansion of the bullet is not. Obturation is the same as obstruction, you cannot obstruct a bullet, you can obstruct or obturate a bore...