As we've seen, systematic errors are always "biased" the same way, either making the measurement consistently smaller or bigger by about the same amount.
In this case, the stopwatch is started very close to the time when the ball is dropped, since it takes light a very short time to reach the girl's eyes. However, sound travels much slower than light -- only about 330 m/s, in comparison to 3×108 m/s for light. This means the sound would take a finite (and very significant) length of time to reach the girl's ears, nevermind the length of time her brain takes to register the sound and tell her finger to press the button.
We can do a short mental calculation to figure out roughly how long it will take the sound to reach the girl. If the speed of sound is 330 m/s, and the distance is 100 m, it should take about a third of a second for the sound to reach her... adding that much systematic error to each measurement.