It's not because you still get the +6.5s if you start at the front. Here is the official explanation;
Update 14 - GP Events - Extra seconds added to Race End Series times
When playing the Race End Series, the race timer starts counting as soon as the 3, 2, 1 countdown is over. However, it's only when you cross the starting line that the timer's animation appears on the top left corner of your screen.
Therefore, the time which first appears on the top left timer corresponds to the time spent between the moment the 3, 2, 1 countdown was over and the moment you crossed the starting line and entered sector 1, whereas the top right timers correspond to the sector times and the first sector time starts only when you cross the starting line.
Since these timers don't start at the same time, there is a difference between the two times. This is something that affects all the players, therefore, no player is at a disadvantage.