Countries are phased probably depending on their server bandwidth where the update is downloaded from. If all countries are provided with the update at once, it may cause the server to break causing even more problems. So countries are prioritized as per the user base and market forces. I say market forces because it also depends on what country Samsung's rivals are targetting first. That's why the US always is the first stop for even non-US manufacturers.
In some cases, there may be some localization changes - both software and hardware-based.
