Hello! Your question is very common, and the answer lies in the fundamental difference in how openSUSE Leap and openSUSE Tumbleweed/Slowroll are built and updated.
Leap: Fixed Release (Updating with Patches)
Leap is a fixed-release distribution (like Ubuntu or Fedora). It has a stable system base that doesn’t change drastically. The updates you receive are patches—security fixes and bug fixes for individual packages.
This is why the graphical notifier alerted you: it was designed to look for and apply these patches, which are minor and safe updates to the packages you already have.
Slowroll/Tumbleweed: Rolling Release (Upgrading with “dup”)
Slowroll, just like Tumbleweed, is a rolling release. It doesn’t receive patches. Instead, the entire distribution moves forward as a single “block” or snapshot. An update means synchronizing your whole system with the latest snapshot, which can contain hundreds of new packages.
This is where sudo zypper dup (distribution upgrade) comes in. It’s the only command that performs this full synchronization correctly. Unlike a simple update, dup can:
Upgrade packages to newer versions.
Downgrade a package if the new snapshot requires an older version for compatibility.
Remove packages that have become obsolete.
Add new packages that have become dependencies.
If you were to use zypper update on a rolling system, it would only update a few packages and ignore the necessary downgrades and removals, leading to an inconsistent system with broken dependencies.
This is why the graphical notifier doesn’t alert you about updates on Slowroll: it looks for patches (which don’t exist) and concludes the system is up-to-date, when in fact a dup would bring in an entirely new distribution image.
In short: On Leap, you received notifications to update individual packages. Now on Slowroll, you perform a full system upgrade with sudo zypper dup, and the terminal is the standard and recommended tool for this operation.