Each Canton has about 40,000 people, the size of many cities on Habololy. So in depth knowledge on that scale is a lot.
That said, you will still know a good amount using the knowledge about the city and cantons. For example, you know where all the shrines are, who the leaders of each canton is, the names of all major streets, names of the big shops, etc.
Another example, you want to know the name of some butchers. Your Knowledge (Local) check will give you the top five for the whole city (say DC10). Each canton you pick will give you the list of all butchers in that canton (also DC10). That doesn't mean you have no chance of knowing all the butchers in a canton you haven't picked, it just means the DC is higher (say DC25).
How does that sound?