The film came pre-cut of course but he was surprised it wasn't precision laser-cut to fit the headlight perfectly and it needed to be trimmed around the entire diameter.
Yes, I wondered about that myself but I think they give you that extra bit to help you install it without getting your fingerprints on the actual part that covers the lens. It also gives you some slack in case you get it off center a bit.
It was only about 3/8" that needed to be trimmed on mine and it wasn't all the way around. What I did was wait 24 hours and trim it with the supplied razor knife - took about 5 minutes.