On the Google Calendar app on my Android device (Sony Xperia Z2, running the Android operating system version 5.1), I have an extra system account calendar that appears.
The owner is called SYNCML-ORPHANED and has one calendar named Phone calendar. A solution to fix this behaviour is something I’m still looking for. I already tried removing my Google account and adding it back; or reinstalling the Calendar app manually, but to no avail.
Feel free to add your comments if you have any solution. From my Google searches, it appears that Sony devices might be the culprit here.