Why Debian APT is not listing the same upgrades on two different servers with the same APT config?

Are you sure that both server are simply in the exact same state of level packages ?

Differences can come from deep packages dependancies (maybe a little tool is installed is one server and not the second one), or from a material difference (storage difference, processor, ...)

You can check the entire list of installed packages with apt list in order to compare them !

You could always try clearing your apt cache, then pulling the lists again and checking for updates. It may just be a little dependency holding the update back.

To clean the apt cache, issue the following command: sudo apt-get clean To update the lists, issue the following command: sudo apt-get update

I hope this fixes your issue.