Originally Posted by
chollie
I have read somewhere that it's because most paper is 'organic' - processed wood. Books or piles of paper, particularly thin paper, are both dense and 'organic'. It's supposedly similar to the reason cheese in blocks will get pulled for inspection - it's a dense 'organic' that some xrays/operators can't distinguish from a block of some kind of explosive.
That's my understanding, also. A block of undifferentiated organic material of that sort of density looks like any other such undifferentiated organic material. Cheese, paper, explosives, they can't tell them apart.