The problem is that they they use Hawkeye, which is a computerised system that works out, based on a number of different data points, where the ball is in relation to the lines. Then it generates an image, based on its algorithm.
It's not showing you where the ball actually landed, it's showing where it calculates that it landed.
There's another system they use in some lower level events (such as some WTA 250 events I've been to) called Real Bounce that uses 40 high-speed cameras. These actually generate a picture of the ball as it lands, and then that image is shown on a courtside screen, along with the call as to in or out.
Since you're seeing an image of the actual ball and the actual line, there's not really any doubt about it being right.
With Hawkeye there's a possibility of an error, if the algorithm calculates the ball's trajectory incorrectly.
Having seen it in action, I think RealBounce is a superior system.