Either, there are protected cells in the testes that never expand to the full mutation or there is a reduction in repeat number in some of the male's reproductive cells. This means that all females who do have the full mutation must have received it from their mothers since they cannot receive it from their fathers. Premutation mother to daughter Females have two X chromosomes. Each child has a random chance of receiving one or the other of these two X chromosomes. A female who has one copy of the premutation and one copy of the stable version can pass either the premutation or the stable version to her daughters.
Every daughter has an equal probability of receiving one or the other version.
This means that half of all of the daughters of all of the premutation females will receive the premutation chromosome. However, it does not mean that each individual female will have one daughter with the premutation and one daughter with the stable version. Just as a family with two children could have two girls, two boys, or one of each, a premutation female with two daughters, could have zero, one, or two daughters with the premutation version. Most of the daughters who receive the premutation , will have more repeats than their mother had. The sons also have a 50 percent chance of inheriting the stable version of the FMR1 gene from a premutation female.