Two, things get moved. Archives that were at one time in a central location may be moved to outlaying provincial or regional archives. Unlike in the case above they are theoretically still accessible if one can get another letter of introduction from a local university director and can travel several hundred miles to the new location.
Three, some stuff just gets lost. That is they can not find the file and there is no record of it being ever classified as secret or top secret or moved to another archive.
Four, a lot of stuff before the 1940s is not typed. Instead it is written out by hand and is extremely difficult to read. Even though typewriters existed in the 1920s they evidently were not in widespread use in many parts of the world.