When you have clicked the 'Get Available Interpreters' button on the Interpreters tab of a request you will see under the Distance column a display of buttons which have the approximate symbol followed by a value.  That is the estimated distance from the interpreter's home to the job and is calculated from zip code to zip code.  If you need to have the more precise number, you can click this button and it will get the actual street distance from Google Maps, as shown below: