Using the persistence analysis option affects the probability of failure because of the trace length distribution. What the program does is randomly create a wedge in the slope. The wedge has a random height and the apex of the wedge on the slope surface is at a random location. The program then samples the trace lengths. If the trace lengths associated with the random wedge exceed the sample trace lengths, the wedge is invalid. The higher the slope gets, the fewer valid wedges you have.