Judging by Apple's security updates page[1], they're still releasing standalone security updates for older versions of macOS. Unfortunately, the same is not true for iOS where security updates are rolled into OS updates.
Apple used to release a security update for revision N-1 and N-2 every time the current release got a point upgrade. However, 10.12.0 and 10.13.0 were both released on their own. Only when 10.12.1 and 10.13.1 were released did older systems get patched.
So every time a new macOS release comes out, everyone has the choice between running a .0 release, or running an insecure OS for about a month.
Do you have any clue how to subscribe to security updates only on OS X? Looking for OS updates simply suggests downloading High Sierra, but my work laptop has to stay on El Capitan for dependency reasons.
[1] https://support.apple.com/en-us/HT201222