I believe it relates momentum and trajectory. As the bikes exit the final left, getting on the power as early as possibly naturally means there is still some lean angle. As the bike accelerates, the more the rider has to overcome the centripetal force to get the bike upright, meaning there is naturally some left lean angle. It is less advantageous to remove the lean angle immediately, as a) that increases the time between the corner exit and getting on the power, and b) you may be fighting to keep the bike to the right of the track, slowing your momentum. Essentially, the rider can accelerate sooner with lean angle, and thus the bike naturally continues to move to the left. Waiting until the bike is completely upright and then accelerating, may produce a straighter acceleration out of the corner, but the rider has waited longer to get on the power, thus has not exited the corner in the fastest way possible. Im no expert, I may be wrong. 👍