When you got these results, didn't the evaluator explain them to you? The person who does the evaluation is really in the best position to explain them, because there is more to the evaluation than just the scores. Putting that aside, it looks (from the scores) like your son's receptive language is very good, and it is "only" the expressive language where your son is struggling. While a 7% increase might not seem huge, it really is a very significant increase in his abilities, especially at his age(s). Between the ages of three and six, when your son started speech therapy and now, the expressive language growth in his peer group is enormous, so for him to "move up the ladder" when compared to those peers is a big deal.
Part of the improvement in speech is developmental, but without the intervention of a good speech and language pathologist, the child won't be able to make the most of his developmental growth to maximize his language development. Many children who receive good speech and language services see huge improvement in their language - and many children continue to receive those services for many years. They key is that the services be good ones, and that they change as the child grows, to meet the child's language needs and developmental level.