To expound upon Todd's answer, some of the combo boxes are populated by a system list that can be open or closed. When the list is open, new values that a user enters can be added to the list. City is one of those. These lists can be seen under Administration > Platform > Itemized Lists. They are different than an enumeration, which are defined as part of the data modelling.
....and I just discovered that Martinez IS in fact a city in California in the zip code of 94553, so my advice was terrible, lol. Hey it's not like we can just Google things like this whenever we want...wait, we can? My last excuse just fell apart!