The bottom line is that Office for Mac does not support CalDAV, which is the type of calendar hosted by Google and iCloud. Outlook will only sync calendars with iCal that were:
1. Created in Outlook.
2. Created in iCal "On My Mac".
If a calendar is created "On My Mac", then it is hosted locally (on your Mac) and Outlook can sync to it.
If it's created in Google or iCloud, then it's hosted on a server in the cloud. iCal syncs to that server using CalDAV. Outlook doesn't support CalDAV, so it can't sync with those calendars.