first >> we know that quantum mechanics works with a probabilistic nature so that we can't say " what will happen? " but " what might happen? "
second >> we can ask how quantum mechanical systems evolve in time and indeed we obtain a time evolution operator for state ket so now we can simply answer this question "at time $t$ what is the ket state of the physical system? " and if we measure the probability of existence at that ket the answer would be 1 which means the system is absolutely at that state!
I can't gather these two things in my mind. the first says that we only can talk about probability but the second one says we know exactly what configuration our physical system at. where is my mistake? what am I doing wrong?