Its quite normal i think, they have to make sure its a genuine memory - people who arent telling the truth tend to not supply much details, so its their way of ensuring you are being truthful. It sounds cynical, and they dont do it to prove people are being dishonest, its a protection for you and for them to make sure they are helping you in the best way. Sadly people do lie to therapists, for many many reasons - but a therapist has to be sure at all times, otherwise a lot of damage can be done