Originally Posted by blakesq
The problem is geometry. If you are receiving a serve, and the ball catches the "back" of the line, it is IMPOSSIBLE for the receiver to see the ball in or out. The receiver's line of sight is blocked by the ball, and hence, you cannot call a ball in or out if it catches the back of the line, or if it is just barely out. Thus, you should rely on your partner.
This post is correct, of course, but begs the question, why is Cindy having this concern with this partner out of the presumed many partners she's had? Especially when all of the other lines are being called correctly. My guess is that this player is calling the lines correctly and many other partners call them generously.