It is a proximation. Each zip code gets a latitude and longitude, for which I guess is at the center of that zip code region. Then you can compute the distance from that center to bank location, or radius to be exact. So distance info is very rough estimation.
If you want to know the exact direction to the ATM/bank location, you should use "street address" instead of zip code, the google direction will give you turn-by-turn direction to the bank.
