Planing Engineering Plastics - Engineering plastics are pretty much mostly produced oversize (thicker), the objective of this is so that you can get the nearest thickness and skim it down to whatever thickness you like, and at the same time remove any of those potential surface imperfections along the way.
Find out more from the Engineering Plastics experts at https://www.directplastics.co.uk/news/it-s-just-plane-better