[expat2]

One reason: if the forger were unhappy with his results for some parts of the text he was forging, and had another go at those parts. In that case, those parts would end up in a separate layer. (I should add that I have significant experience in editing imagery for (legal) use on the web, and have practical familiarity with the vagaries of layer generation during such editing).

[Conscience of a Conservative]

But isn't the whole point of using layers that you can work in one layer for things like (in the case of forging a birth certificate) the entries you're altering/forging, but leave things like the basic form elements alone? So that if you make a mistake or are unhappy with the results of your forgery, you can go back and have another go at those parts without altering the basic form elements? It doesn't make sense that a forger would go through the trouble of working with all of these different layers, only to place elements he's forging in the same layer as basic form elements that remain the same in any version of the document, forged or not.

Also, in the case of the "Name of Hospital or Institution (If not in hospital or institution, give street address)" line I cite above, both the form box in which that text appears, and the box above it are typewriter-entry boxes, with the typewriter entries coming nowhere near the "Name of Hospital or Institution . . ." text. Even if the forger had placed that text on the same layer as elements of the document that were being forged, it seems implausable that any mistake would have affected that text.