Unfortunately, city mappings are a shared system resource for all Adobe clients, this isn't something you can fix yourself.... In the past, we have found some opposite issues.. where Adobe was correct and GA was wrong (years ago, someone forgot to configure bot blocking rules on one of our suites and we started getting a lot of Data from Bing... we know that Bing is located in Richmond, VA and Adobe had the location pegged... GA on the other hand was way off)...
That said, the locations are really only as good as the IP lookups for your traffic... if you are using IP obfuscation or hide the last octet, this will have an impact on your reported locations.
IF however, you believe the location mapping to be in error... the IP addresses are available via a Data Warehouse extraction (I suggest a column for IP, another for the City, so that your resulting CSV will be readable, and then a metric like UVs - sort by metric to get the highest first)... you can pull these out, perform your own spot checking IP lookup and see what you come up with... If you find data that supports the city being wrong, I suggest opening a ticket with client care, providing details to support your position that the cities are mis-mapped. Hopefully this will get the to check their mapping and ensure better results for everyone.
