I took a look at your account to see your Sign Up Form. What seems to be happening is when someone signs up they are required to add a Company Name to the form. This creates a Company Contact. So for each sign up 2 contacts are being added. The difference in the numbers can be people who are already on your list that are attempting to sign up. If they are on your list but did not have a Company Name listed, when they fill out the form and add Company Name, they create that contact. In order for that contact (the email address being added) to then be added to the correct list in your account they would need to complete the Update Profile Form.
When an existing contact goes through the Sign Up form, they get a message saying looks like you are in the account, do you want to Update your Profile. From there the Contact has to click Yes. An email is then sent to their address where they need to click "Update Profile." Once they fill out the Update Profile Form, they would then be added to the list(s) that you have selected on the Update Profile Form