That would assume that the letoff occurs at the same instant as the primer ignites. But it does not. Sear release or letoff occurs first, after which the hammer falls, after which the firing pin crushes the primer, after which the primer ignites, after which the primer ignites the powder, after which the powder burns, after which the bullet is pushed out of the case. Only after the bullet is almost all the way down the barrel does it achieve the muzzle velocity of the 1000 fps in your illustration.
The critical area for trigger letoff and overtravel is the period between letoff and primer ignition, and it is in that period that the gun can easily move off target due to overtravel, vibration from a heavy hammer fall, or simple movement of the shooting hand.
All those things wouldn't matter, or could be adjusted for, IF they were exactly the same each time. But in areas where the human anatomy is involved, things are not going to be exactly the same each time, and that is where accuracy is affected.