I know this can be quite confusing - 38139 is a Germantown, TN zip code for the postal service. A small area of Collierville for now, receives it's mail through this post office. Also, the districting for schools (since both areas are in the "county school district") are in the same boat.
For instance, Houston Estates and Almadale Farms are located in the city of Collierville - right on the cusp of Germantown. For NOW, they get their mail is delivered through the Germantown post office. As this area grows - at some point, a new zip code will be issued for just this area. Same goes with drawing the lines for school districts. It's not a matter of if, just when.
The same goes for Lakeland, Arlington and the northeastern part of Bartlett. They all use 38002 zip code. Those three areas mentioned are all incorporated cities within Shelby county (and very different), but share a zip code, some school districts, fire and water service, etc.
If you are looking for a certain school district or area, please enlist the help of a Realtor that can navigate you through the process. Something this important shouldn't be undertaken on an assumption.
Let me know if I can be of any further help and welcome to the area!