I use Kubuntu and I use to update the system with the terminal running sudo apt-get update and sudo apt-get upgrade.
The system download and install packages. However, sometimes after updating the system still ask for updates and when I open the menu it shows pendant downloads at Discovery. Frequently the updates are kernel updates.
Why Ubuntu does not update all packages?
sudo apt update means to update the repository information, not to download anything. Basically a list of available packages and their versions will be downloaded and stored.
Then you might "update and install" existing code by executing:
sudo apt upgrade.
Still some packages may be held back. (e.g Kernel). To get all the new stuff you can issue
sudo apt dist-upgrade
As user535733 mentioned in the comments there is a 3rd possibility to upgrade:
sudo apt full-upgrade
See here for a full explanation of the two upgrade commands
Some packages might be kept back even when you try to upgrade. You can check the available updates and update those packages separately.
apt list --upgradable
sudo apt install --only-upgrade <available packages for update>
