Yes, I cannot upvote this enough. Stuttering is extremely complicated and very little is known to what actually causes it to this day (although it has been noted that more than 31 genetic mutations could cause it). Speech therapy is a science, so the psychologist are literally practicing science. Think of each therapy session as an experiement, seeing what helps and doesn't help. This could be normal stuttering techniques, medication therapy, anxiety therapy, a ton of different things. Sometimes all of the above is used to try and attack each angle at the same time.