What Brian is trying to tell you is that there are different email addresses for Production and for Sandbox Email to Case. Cases are being created in Production because you are using a production email to case address. You need to actually create a completely different email to case address in your sandbox and use that one to test. Then when you go live in production, you will get a new one to use for production cases.
Most likely you have setup an on-demand email address and handed that off to your email admins to create a forward for. To test your email to case in your sandbox org, simply just send an email to the Email Service Address in your sandbox org.