Why you do not get 4.3.8966 on a sync depends on who your affiliate it. But that's rather not all that much of a problem as you can use the following link to download the firmware and side load it to manually update.
You just unzip the contents to the .kobo directory and eject your device and it will update. Just make sure you are charged before doing this.
https://geek1011.github.io/KoboStuff/kobofirmware.html