Why box JWT token doen't follow the standard JWT structure?

I am using the Box .Net SDK to get UserToken, and token works fine when I am using it with Box APIs. But while looking more in JWT details https://jwt.io/introduction/ I see that the JWT should has three sections seperated with (.) Header.Payload.Signature.

By calling 

 boxJWT = new BoxJWTAuth(boxConfig);
token = boxJWT.UserToken(_boxUserId);

I am getting a token with only one section. how can I get the standard JWT token ? any ideas