When the
JSP file is processed to generate a Servlet, the JSP processor makes one which conforms to the Servlet API specification, and thus includes a "service" method. Depending on the particular JSP implementation it can do that by inheriting from some generic JSPServlet class or by actually coding it all in the generated servlet. This method does some stuff, then calls a _jspService method. which is also generated in the same way.
As far as the container is concerned, there is only a "service" method, and that's what it calls.