You received a meeting appointment where the organizer is a C4C contact and you as a C4C user has Outlook Server Side provisioned, however this meeting didn't sync from Outlook to C4C.
SAP Cloud for Customer
This is currently the system expected behavior. In current implementation automatic synchronization skips the events where C4C contact is an organizer.
You need manually categorize the such appointments in order to sync from Outlook to C4C.
C4C Contact, Outlook Sync, Server Side. , KBA , LOD-CRM-GW-SCC , Invisible CRM - Smart Cloud Connect Solution , How To