There are a few different types of synopsis writing services. The first is through the use of a "synopsis generator." This tool will take a basic outline and generate several variations. For example, it might generate a short description of the basic premise of the story. It could even explain what emotions or attitudes must be evoked in order to effectively tell the story.
Some services specialize in this process exclusively. One service, in particular, has a group of writers available for only a select number of projects. The writers are highly-trained writers, but they are not freelancers. Their services are valued at well over six figures. Companies that provide such services will have a high overhead.