Some Google Calendar apps and programs have the option of making an event tentative. Additionally, some Android phones experience a bug where every event you add through the phone is automatically set tentative, with no way to adjust it. The ISN, by default, ignores these events as to not interrupt your schedule with not-sures and might-gos.
If you need the ISN to sync your tentative events you will need to toggle the setting on. Head to Settings --> Office Settings, then under What we see on the ISN, click on Google Sync:
Check the box that states "Synchronize tentative events from my Google Calendar (only works when one way sync option is OFF)" and click Save Settings:
The next time ISN starts to synchronize with your Google Calendar it will allow tentative events to be added.