Gfilui

When I login to my Ubuntu VM, I get the following message informing me that 8 packages are upgradeable:

Welcome to Ubuntu 16.04.3 LTS (GNU/Linux 4.4.0-112-generic x86_64)

 * Documentation: https://help.ubuntu.com
 * Management: https://landscape.canonical.com
 * Support: https://ubuntu.com/advantage

8 packages can be updated.
0 updates are security updates.


Last login: Mon Jan 29 14:55:34 2018 from 172.16.117.1

However, when I issue an apt list --upgradeable I get 26 packages.

$ apt list --upgradeable 
Listing... Done
cloud-initramfs-copymods/xenial-updates,xenial-updates 0.27ubuntu1.5 all [upgradable from: 0.27ubuntu1.4]
cloud-initramfs-dyn-netconf/xenial-updates,xenial-updates 0.27ubuntu1.5 all [upgradable from: 0.27ubuntu1.4]
cpp-5/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
distro-info-data/xenial-updates,xenial-updates 0.28ubuntu0.7 all [upgradable from: 0.28ubuntu0.3]
g++-5/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
gcc-5/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
gcc-5-base/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libasan2/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libatomic1/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libcc1-0/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libcilkrts5/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libgcc-5-dev/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libgnutls-openssl27/xenial-updates 3.4.10-4ubuntu1.4 amd64 [upgradable from: 3.4.10-4ubuntu1.3]
libgnutls30/xenial-updates 3.4.10-4ubuntu1.4 amd64 [upgradable from: 3.4.10-4ubuntu1.3]
libgomp1/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libitm1/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
liblsan0/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libmpx0/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libquadmath0/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libseccomp2/xenial-updates 2.3.1-2.1ubuntu2~16.04.1 amd64 [upgradable from: 2.2.3-3ubuntu3]
libstdc++-5-dev/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libstdc++6/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libtsan0/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
libubsan0/xenial-updates 5.4.0-6ubuntu1~16.04.6 amd64 [upgradable from: 5.4.0-6ubuntu1~16.04.5]
python3-update-manager/xenial-updates,xenial-updates 1:16.04.10 all [upgradable from: 1:16.04.7]
 update-manager-core/xenial-updates,xenial-updates 1:16.04.10 all [upgradable from: 1:16.04.7]

Why the mismatch?

They run at different times.

Items are uploaded into the repositories at all hours (not once daily), so if you compare --upgradable at different times of day, you will get more and more results as the day goes on.

Apt does NOT check for packages upon startup - that would needlessly delay startup. The login message (part of Message Of The Day [motd]) is generated during the daily run of apt-daily...during some previous session (like yesterday). The text is generated and saved for your next login. It's meant as a reminder, and that lack of accuracy is a well-known minor issue.