Is there a difference between the census population and the actual number of resident in your capital or major cities? If so, how big is it ?

In Greece it’s very widespread that Athens has more likely 5 million people than 3,7 that the census shows for the metropolitan area and that’s because many don’t change the area they are registered. They even own their voting rights outside of Athens sometimes even though they’ve never or barely step their foot on their towns for decades.