First, as you probably know, the times given don't give actual time, just road time and doesn't include any stops and they assume "normal" traffic (whatever that is).
Second, some travelers reporting in here have said that, for them, at their driving speed, that the driving time took them more time than given in Google. One explanation is they were going slower than Google assumed. Another reason is they may have made stops, even just short ones, and consequently weren't really doing an "apples to apples" comparison with Google, which gives strictly road times, times that tires are rolling.
We probably only hear from the people who are complaining because they found the Google times "too fast." I doubt that we hear too much from people who found the Google times pretty accurate.
Maybe we will hear from those people now?