In the Google Console it shows me version 1.13.12 as available:
1.13 is the stable channel, when I check the release notes for it at https://cloud.google.com/kubernetes-engine/docs/release-notes-stable the latest version it mentions is v1.13.11-gke.14 which matches the hint on the screenshot that my version 1.13.11-gke.14 is the current one.
But when 1.13 is the stable channel and 1.13.11 is the current stable release, what's then 1.13.12, which is obviously newer than 1.13.11?
If I were to harbor a guess, the releases appear to be related to the Nov 4 - Nov 13 outage.
The full release notes page shows that 1.13.12-gke.13 has this change:
This version uses cos-u-73-11647-329-0 which fixes the known issue that may cause kernel panics, previously reported on November 5th, 2019.
and 1.13.12-gke.8 has this change (which was released concurrently with 1.13.11-gke.14, on the stable channel):
This version includes a fix for a known issue in the COS kernel that may have caused nodes to crash.
Based on this page, it seems that these releases are actually "No Channel" rather than "Stable Channel", so are available to clusters not using a release channel, thus the notes would not appear on the Stable Release Channel release notes.