Dp they have white out
Here is some info on how/why .pdf scanned docs will have layers. There are two reasons, OCR- Optical Character Recognition and MRC- Mixed Raster Content. The former explains the text layers, the latter explains the image itself broken into layers as well as what some call the “white out” look. There are even image samples in the second link that are almost exact mirror images of that “white out” effect.
http://blog.nitropdf.com/index.php/2008/09/10/text-works-ocrd-scanned-pdf-files/
http://image.unb.br/queiroz/papers/icip08mrc.pdf
I suggest studying these before continuing this line of ‘proof’ regarding the layers.