Padcast syncing issue

The podcast on my 5s does not sync right with the iTurn 11.1.4 on my iMac, the episode that I have listened already on iPhone will reappear in iTurn. Is this right? I thought 5s and iMac will understand each other(I turned the sync option on on both devices) about which episode has been heard so the iMac would not download that episode agian.

Thanks in advance for your advise!

Yudan