I'm glad you mentioned "convention." A lot of things concerning navigation are by convention. As a cartographer I can assure you that "North" was not always "up." Many older maps have what we call the North Pole at the bottom of the sheet! If you think about it, it really doesn't matter since all the really important relationships between maps and compasses are independent of orientation on a piece of paper. It's orientation in the "real world" that really matters. To be perfectly correct your compass doesn't really point "at" anything (like the so-called magetic north pole) but, instead, aligns itself with the local magnetic fields orientation where you're standing. As you move around the earth to various locations and plot these lines of orientation they are referred to as isogonic lines of declination meaning a line on the earth where the variation of the compass from true north is the same all along the line. The one isogonic line where the variation from true north is exactly zero (0) is known as the agonic, or a line describing all those points on the surface of the Earth where true north and magnetic north are the same (for the time being!) Also, bear in mind that the magnetic poles of the Earth are constantly moving (albeit slowly). Pole Drifting
I like the explanations given to your dilemma as being most likely correct. One other, more remote, possibility is that the faulty compasses were stored near some fairly powerful source of magnetism for a period of time and the needles were re-magnetized. This could be either a metallic magnet or possibly, an electrical field, such as a motor.