Restoring iOS devices and app login credentials

Mostly curious here: I have about 50 apps on my iPhone. About 20 of these apps require login credentials (e.g., Twitter).

I have to restore my iPhone periodically due to syncing issues with iTunes (that's another story). When I do the restore, I am restoring from an iCloud backup. My backups are encrypted.

When the restore is complete, I've noticed that I can use some of the apps without having to re-enter the app-specific login credentials. I guess iCloud was able to back up the credentials and restore them for these apps. But for most apps, I find that I have to re-enter the login credentials in order to use the app. For these apps, it seems that iCloud did not back up the credentials.

It's tedious re-entering login credentials for a number of apps, so I'd like to understand how iCloud is supposed to work. Why does iCloud restore some app login credentials and not others? Is there a setting somewhere that tells iCloud whether to backup (and restore) app-specific credentials? Thx

FYI that I do not use iOS Keychain, I have it turned off.