My understanding, only from reading here on the forum, is that there is also some delay in processing the ultrasonic sensors. If you were letting cars pass too fast, whatever that is, they may not have been solidly detected.
It was disappointing to see the article Elon linked on Twitter ( Tesla vs Texas: a 700-mile road trip in a new Model X ) say:
"To change lanes you simply indicate. The move over is smooth and faultlessly observed. If there’s a car gaining on you in the outside lane Autopilot will judge the closing speed and make a call with cool logic, as you would. If it decides to go, the Tesla steers out and up to speed smartly. As you would. Rival systems I’ve tried are wobbly gimmicks by comparison. Tesla’s begs your use whenever possible, the double-pull soon becoming instinctive."
Not sure what these guys were seeing, at least one earlier article said the same thing. Despite the fact that there's nothing but ultrasonics looking back to see that closing car. No wonder the "average" person thinks Autopilot can do everything.
Actually my experience changing lanes using autopilot is exactly as described. This thread has been about the poor blind spot detection of the Tesla when driving the vehicle manually.