When the Buffer tool is run on a feature class in a state plane feet coordinate system using the geodesic distance calculation method and a buffer distance that is not feet (for example, miles), the tool returns an incorrect measurement as output.
This is a result of BUG-000083937. This issue only occurs when the input data is in a state plane feet coordinate system.
Download and install the ArcGIS 10.3 (Desktop, Engine, Server) Geodesic Buffer Method Patch. Follow the instructions provided on the page.