I changed my opt-in form, why is my old double opt-in confirmation email still going out?

When you make changes and update your Custom Opt-In forms, you must also change the confirmation email. The email is not part of your actual opt-in form, but instead is a notification email. To change this you can go to My Account (top right corner) > Notification Templates > Optin Confirmation. Make sure you click the orange save button.