Like most, Ducati speedometers are set-up to read optimistically to avoid law enforcement issues. The speedometer error for a 998 with a 70-section was measured against a GPS standard by one owner to be:
40 998 = 37 GPS
50 998 = 46 GPS
60 998 = 56 GPS
70 998 = 65 GPS
80 998 = 73 GPS
90 998 = 82 GPS
100 998 = 89 GPS
So for example, after a one hour trip at an indicated 80mph he would have traveled only 73 miles but the odometer will read 80 miles. The odometer always records more milage than the actual mileage traveled. The greater the difference, the less accurate the clock.
So if your friend’s odometer reading is lower than yours for the same trip his odometer is more accurate than yours.