My guess is that the line was made during the ID L's creation when the cover plate of the Tama was molded, so technically, it might not be a scratch. I wouldn't consider the line a defect or imperfection; it's just part of the product making process.
Sometimes the factory molds for plastic items affect the end products. It's possible that the mold for the ID L model consists of two pieces and that white line indicates where the two pieces meet in the middle. It's probably more noticable on the ID L models because of the somewhat transparent plastic that's used for the plates. It can be more difficult to notice details like that with a solid plastic (which many Tamas are made of).