Thank you for reaching out to the API Support team here at Constant Contact.
There are a few possible causes for a discrepancy in contact counts on lists. I'd like to get a little bit more information about this to help you narrow it down. For privacy, please send an email to firstname.lastname@example.org with your username and the details of the call you are making to our API when you see this. We'd be happy to dig into this for you.