Why is my current 18.04.3 system not running the same kernel as a new 18.04.3 system?

Solution 1:

Ubuntu 18.04 LTS users have two software stacks to choose from, maximum stability with the original software stack (4.15 kernel), or the moving HWE (hardware enablement stack) which uses the 18.10, then 19.04, 19.10, and eventually settles on 20.04 software stack.

https://wiki.ubuntu.com/Kernel/LTSEnablementStack

Upgrades from prior releases using the stable option (4.15), as do installs using 18.04, or 18.04.1 media. They can opt to enable HWE though.

Installs using 18.04.2 or later media start with HWE enabled.

18.04.2 uses the 18.10 software stack. 18.04.3 uses the 19.04 software stack. 18.04.4 uses the 19.10 software stack (currently in testing). until the stack becomes static using the 20.04 base kernel.

From the provided link, to enable HWE use

DESKTOP
 sudo apt-get install --install-recommends linux-generic-hwe-18.04 xserver-xorg-hwe-18.04 

SERVER
 sudo apt-get install --install-recommends linux-generic-hwe-18.04