Should Google Map give identical results as the Distance Matrix API for the exact same origin/destination pair entered with GPS coordinates?

Question

I have noticed small, but substantive discrepancies for identical queries entered at the same time using Google Maps and via the Distance Matrix API, with origin/destination pairs in the GPS format, a departure time of Now, and use of my API key linked to a paid account. These are for the distances (often off by a few miles) and the DrivingTimeInTraffic (off by as much as 15 minutes). Is this expected behavior, for example, due to latency between traffic updates used by the two methods, or is there something more fundamental that is causing the discrepancies. The same behavior is shown for future departure times, with no apparent relationship between the range of driving times sometimes shown by Google Maps and the pessimistic/best_guess/optimistic times that are returned in the JSON string by the API. I'm coding in python 3.4.4, although that is not likely relevant since the issues are the same when the API query string is pasted into the browse manually. Any assistance will be greatly appreciated.