This one is more of a interface issue than an acutal bug. I'll try to explain it:
There is a difference between your De Jure kingdom (concerning the law) and De Facto kingdom (in practice but not necessarily ordained by law) . You can see the difference between them by selecting different map modes. Some laws are concerning your De Jure kingdom while other laws are concerning your De Facto kingdom. So, when voting for laws everybody that are concerned by the law will get to vote. Sometimes if appears as if people from abroad are voting for your kingdom but in fact: They vote for the law that concerns them.
This gives the game a lot of flavour, historical accuracy and complexity but is very difficult to show in our interface. I know... I have reported this very issue couple of months ago until designers explained to me how it works.