If the testing was done on a clean-installed environment, and not on a device that gets updated, that would explain the entire situation. Clean install via a firmware image, and taking an existing install of 4.1.2 and then upgrading to 4.3 are two VERY different things. Take apps on top of that, launchers, keyboards, anti-virus, etc, and there is plenty of room for problems.
Now, if a factory reset after an update still has problems, it may still be due to the upgrade process, rather than a clean install, so it is still possible testing was done, but not on the critically important "update for existing devices". Ever see the difference on a Windows based machine between a clean install of Windows, compared to an upgrade? A clean install may be PERFECT, while the upgraded result may have a ton of problems.