By default, when regenerating parameters, the parameter system will not allow repeats of the same value.
In this example, no repeats is turned on. When you regenerate these parameters, both $a and $b will be chosen from the set {2,3,5,7}. Without "no repeats" there is a one in four chance that $a and $b will return the same value. This is rarely what is intended when you are defining a question. With "no repeats" enabled, you are guaranteed to have different primes selected for $a and $b