An extra sticker (which is supposed to be removed at step one) was designed to help fix the position of the screen protector, but it turns out to be the loophole of the whole design as it is exactly where the dust can easily find its place.
Also, this screen protector tends to be kind of "sticky" so that it can easily catch more dust than a glass protector does while it is more difficult to remove any dust attached to the protector. An additional star is given just because of the ease of assembly, which outperforms many cheaper counterparts; if you are an expert in assembling screen protector, this should be your very last choice.