If you check the numbers given by Google you will typically find it figures an average speed in excess of 60 mph. So basically they are calculating actual time spent on road, not allowing stops for fuel, food, rest stops, etc. Might be valid for short distances with no stops. Might even be valid for longer distances with some stops if you drive in excess of posted speed limits. But generally speaking for longer drives with typical stops I have found it takes longer than google estimates. I generally use google and account for the above.
The other site must use a different calculation technique.