One of the most old fashioned and out dated aspects of cycling. Pinning on a number is an art form.
Pinning on a number badly instantly signals you out to everyone as a little bit of a whopper, whether that is at a road race or a time trial. If it is a time trial, you may as well just go home. The amount you will be judged will massively out way any watts your legs put out on the road.
Learn to do it right. Four pins horizontal across the top, three vertically down the side. Job done.