Why would a Web Service return a variable in a format that needs conversion?

More specifically, I cam calling a web service via a console application. It requires a login function to be sent via the service and receives a login sessionId that is active for x minutes. The sessionId needs to be a long variable for any of the service defined functions to be utilized but it the service returns a string? Is this a common thing that web services only return strings? Any explanation would be helpful.