Mombasa was TECHNICALLY belonging to Zanzibar (no, it was not IN Zanzibar. Zanzibar is an island and they owned a 10 mile strip of land in the Coast Province in Kenyan terroritory that they LEASED to the British for use in the Dominion Kenya. In order for Zanzibar to gain it’s own independence, they had to cede Mombasa to Kenya.
This is like you renting a house and using that address as your Home Address. It IS yours as you are paying for the use of it, although it is deeded in someone elses name.