How did I get 4.18 Kernel

I have two 18.04 LTS systems, both upgraded from 16.04 LS with do-release-upgrade.

The older upgrade'd system has 4.18.0-21-generic kernel. The most recent(today) has the 4.15.0-51-generic kernel. I believe both should have the 4.15 kernel.

The 4.18 kernel is dated May 15/16.
# ls -l /boot | grep '4\.18'
-rw------- 1 root root 4270411 May 15 07:31 System.map-4.18.0-20-generic
-rw------- 1 root root 4268316 May 16 06:14 System.map-4.18.0-21-generic
-rw-r--r-- 1 root root 218359 May 15 07:31 config-4.18.0-20-generic
-rw-r--r-- 1 root root 218350 May 16 06:14 config-4.18.0-21-generic
-rw-r--r-- 1 root root 43019427 May 24 06:54 initrd.img-4.18.0-20-generic
-rw-r--r-- 1 root root 43181829 Jun 11 21:13 initrd.img-4.18.0-21-generic
-rw------- 1 root root 8555920 May 15 07:39 vmlinuz-4.18.0-20-generic
-rw------- 1 root root 8552184 May 16 07:52 vmlinuz-4.18.0-21-generic

My written log does not show installing the 4.18 kernel or updating it.

Looking at /var/log/apt, I see

Start-Date: 2019-02-15 08:22:56
Commandline: apt dist-upgrade
Requested-By: tomdean (1000)
Install: linux-modules-4.18.0-15-generic:amd64 (4.18.0-15.16~18.04.1, automatic), libwayland-egl1:amd64 (1.16.0-1ubuntu1.1~18.04.1, automatic), linux-headers-4.18.0-15-generic:amd64 (4.18.0-15.16~18.04.1, automatic), linux-headers-4.18.0-15:amd64 (4.18.0-15.16~18.04.1, automatic), libllvm7:amd64 (1:7-3~ubuntu0.18.04.1, automatic), linux-headers-generic-hwe-18.04:amd64 (4.18.0.15.65, automatic), linux-image-4.18.0-15-generic:amd64 (4.18.0-15.16~18.04.1, automatic), linux-image-generic-hwe-18.04:amd64 (4.18.0.15.65, automatic), linux-generic-hwe-18.04:amd64 (4.18.0.15.65, automatic), linux-modules-extra-4.18.0-15-generic:amd64 (4.18.0-15.16~18.04.1, automatic)

What triggered the installation of the 4.18 kernel?

Tom Dean