You might have noticed that when you geo filter for contacts that you have some contacts that look like they should have been included but were not. For example, you have 339 contacts with "Springfield" addresses but only 287 of them show up in your filtered list. What's going on?
What you've run into is the discrepancy between mailing addresses and municipal boundaries.
Mailing address city names provide no guarantee that the address is within the city or town limits of the named city. We'll use Alexandria, VA as an example.
Alexandria: city and mailing address
Below is a map showing the outline of the city of Alexandria, VA. The city is a distinct political entity bounded by Arlington County to the north and Fairfax County to the west and south. The city is shaded in blue. The two dots to the near west of the city, and the three dots farther to the south represent real "Alexandria, VA" addresses which are nonetheless in Fairfax County, VA.
If you were trying to activate your advocates to contact their city council members for an issue in the City of Alexandria you would want to reach out to advocates who are actually in the city boundaries of Alexandria, regardless of what their mailing address lists as their "city"!
Beyond the one city
This is one example, but the "issue" crops all over the country, in both incorporated and unincorporated areas alike.
Many addresses outside of the city limits of Cleveland, OH use "Cleveland" as their mailing address
The cities of West University Place, TX and Southside Place, TX are surrounded by, but politically distinct from the City of Houston, however most addresses use "Houston" as the mailing city
Addresses in the 90210 zipcode use Beverly Hills, CA as their mailing city though only roughly half of the zipcode overlaps with the Beverly Hills city limits