To be fair, I don't necessarily rely on the lines to park my car.
What I'm guessing you're seeing (and my wife complains about this too), when you look at the backup camera, it's not centered over the bumper. So after you park, if you count the pixels on the screen from the lane edge to the car on one side it'll be different than the lane edge to the car on the other side due to the distortion of the ultra wide lens and not being centered over the bumper. That's not directly related to the "lines" that Tesla draws, and there is no "fix" for this.