Because the front sprocket does not have the benefit of pivoting on the same axis as the swingarm, the length of the lever is not static as it moves. The lever is longer as it travels up or down, with its shortest being a straight line drawn through the front sprocket (center), the swingarm pivot, and the center of the rear axle. This being said, a perfect adjustment would be the bike straight up and down, with the rider on the bike.
This is not neccessarily the slack that Honda tells you to place in the chain because they tell you to adjust it while on the kickstand therfore making it the standard that you start your adjustment at. This is only an explanation of a small part of the physics involved when thinking about how it all works.
Do it by the book and you are always safe and as Oldman said, never too tight if you were to err either way.