So what's the difference between these updates and the automatic behind-the-scenes thing that is supposed to keep us all safe without us having to do anything?
It's unclear when Apple decides to issue Rapid Security Response (RSR) updates.
You can read more here:
About Rapid Security Responses for iOS, iPadOS, and macOS - Apple Support (CA)
Rapid Security Responses deliver important security improvements between software updates.
And the list of releases here:
Apple security releases - Apple Support (CA)
This document lists security updates and Rapid Security Responses for Apple software.
The last RSR update was mid-July. Sounds this exploit fix in today's update should have been issued as an RSR, too.