Photoshop - contract selection doesn't work for the whole document

Question

Select -> Modify -> Contract works as intended whenever I select an area within the bounds of the document, but for some reason it doesn't work for the whole document, i.e. after selecting all (Ctrl + A). Selecting all seems to be a selection like any other, so why can't I modify it as I wish? It's strange especially since I just want to contract it, so it doesn't involve anything outside the document bounds.

Answer 1

It turns out I should have checked the "Apply effect at canvas bounds" box. There is such an option in all but "Border Selection" dialog boxes from the Select -> Modify menu.