The drive times using historic traffic depends on depart time. Historic traffic speeds differ by time of day and by road segment. Defined speeds do not differ by time of day and do not differ by road segment. (For example, all urban primary roads in NJ will have the same default speed for trucks/medium trucks for all times of day.) This will create scenarios where defined speeds are higher/lower than historic speeds.
Traffic data is road-segment-specific, whereas defined road speeds are state-wide averages. Specific road segments of the same road class in the same state can be up to +/-15 mph faster or slower than the state-wide average for that road class.
So traffic data is still more precise then defined road speeds, even though defined road speeds are improved in each version of PC*MILER.