It is a limitation of the bomb's ability to alter course.
In many cases, you can get away with this is the delivery is at the right altitude...if the drop is from too high or too low, even though the laser is tracking the moving target, the bomb's guidance system cannot make enough adjustments to follow the target.
USUALLY, a drop from 5-6000 feet is low enough that the vehicles wont travel too far during the bomb's fall, and high enough to allow correction by the bomb's fins.
LGB's aren't powered, so there is only a limited amount of maneuvering they can do.