Posts which you have never marked as paid show up as already paid, making the author balance incorrect (or even negative).
This problem probably happens because you use a duplicate post plugin: when you duplicate a post, depending on your settings, even the post custom fields could be copied. Post Pay Counter payment history is stored in a (hidden from editor) custom field, so that if you duplicate a post, even the payment history gets copied.
To prevent this, you need to edit the duplicate post plugin settings, adding to the fields exclusion list _ppcp_payment_history. In the following screenshot you see how Duplicate Post should be set up (look at the bottom field, under Do not copy these fields).
To fix the issue for existing posts, you need to delete/overwrite the _ppcp_payment_history meta. This will most likely require direct database access or an advanced custom field editor.