ANSWER
The Cities tab within Geo Tracking lists the cities in which the email has been opened [not the home / work / preferred city listed on the subscriber's contact record; but again, the city in which the email was opened ] and the corresponding number of contacts for each city, which you can browse by clicking the total in the appropriate column.
The [total] number of contacts listed represents the number of contacts who clicked in each city. A contact may have opened your email in more than one city, in which case they would appear in more than one count [meaning, the combined city counts may add up to more than your total contacts].
