Why some events are not being deleted?

By default, if you delete an event from Notion, we just un-sync it instead of permanently removing it from Google Calendar (same vice-versa).
This will let you delete it manually and prevents mistakes of losing data that you have attached to that event.

You can use the view filters to skip deleted entries:

• Where "Notion Automations Events Data" doesn't contain "removed"
  and "Notion Automations Events Data" doesn't contain "ignore"
  

If you want to allow event deletion anyway:
Go to the "Sync setting" > "Allow event deletion":