It makes sense to me, for whatever its worth, that the expanding gas in the barrel is limited by the speed of the bullet and travels down the barrel at the same speed as the bullet. Once the bullet leaves the barrel the gas is able to expand faster than it was in the barrel so then it begins to overtake the bullet. If the bullet is perfectly shaped and the pressure distribution in the gas "bubble" is perfectly uniform there would be no outside the barrel effect on the bullet. But nothing is perfect. If a muzzle brake/air stripper can lessen the amount of gas following the bullet then there should be less effect after it leaves the muzzle.
Any thoughts on this reasoning?