The difference in the post-update experience depends on whether the iPhone is receiving a major iOS upgrade or a minor update within the same major iOS version.
If an iPhone is upgraded from iOS 26 to iOS 27.2, it may display the “Hello” screen after restarting, followed by the usual setup or welcome experience associated with a major iOS upgrade.
However, if an iPhone is updated from iOS 27.0.1 to iOS 27.2, it will generally restart and return to the Lock Screen, displaying the “Software Update Complete” message rather than the “Hello” screen.
In your example, you mentioned that both iPhones were updated from the iOS 27.2 Release Candidate (RC) to the public release. If that is the case, it would be useful to confirm the exact iOS version and build number installed on each device before the update, as both devices should ordinarily have a similar update experience when moving from the same RC build to the corresponding public release.
To check the currently installed version, go to Settings → General → About on each iPhone and look at the iOS Version entry. Tapping the version number, where available, will show the build number as well.
The key distinction is that a major iOS upgrade may present the “Hello” screen, whereas a routine update within the same major iOS generation generally returns you to the Lock Screen with a “Software Update Complete” message. The screen displayed by itself, however, is not definitive proof of the previously installed version.