In some scenario, the ProxySG might have aging sessions with some particular server. Some connections might be older than two weeks, and still active.
Active Sessions can be viewed in Management Console > Statistics > Sessions > Active Sessions.
You can add the following to your CPL policy, then generate new connections, and close them out, and see if they are still in Active Sessions.
From 000008893: Delete on abandonment not working on ProxySG when client prematurely terminates a download. In active sessions, server bytes will continue to increase until download is complete.
This unexpected behavior can happen when the ProxySG has http.allow_compression (yes) in the policy. So change this setting to "No".