When is an EmailMessage record really created?

According to these help articles, an EmailMessage record should be created when a message is sent from Salesforce.  I interpret that to include the email composer but not an email sent from Outlook and captured with EAC.  However I'm finding that EmailMessage creation is only about 1 out of 20.  Does anybody have a deep understanding how this works?

Referenced help articles:

https://help.salesforce.com/s/articleView?id=000381944&type=1

https://help.salesforce.com/s/articleView?id=sf.emailadmin_enhanced_email_considerations.htm&type=5

https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/sforce_api_objects_emailmessage.htm