I could answer that question for ya -- it's pretty easy to see if the ball is touching the line or not as it's rolling. If you see brown dirt between the ball and the line, then it's foul. But like anything else in umpiring, it's the angle that you have as you're looking at it. The HP umpire should be straddling the line looking straight down at it. It's possible that Garver's positioning blocked out the umpire and he may have had to move a little off that normal angle and that may have skewed what he saw, especially if he moved to the foul side of Garver to look at it.