Censorship "skips" (again — sorry!)

My first post on this topic got sidetracked when I realized that I had
made a boo-boo, writing "Dublin" instead of "Belfast." With your
indulgence I'd like to start again. Here is the question again, now
corrected:

I have a few international covers in my WWII collection, from various
countries, which were not censored. In some cases, de facto censorship
was carried out via currency export control inspections, but other
covers show no evidence of even cursory censorship. I have two British
covers, for example:

— One posted from London to the U.S. on September 2, 1940, was *not*
censored.

— Another cover, posted two days later from Belfast to the U.S., *was*
censored.

Does anyone know whether censorship was supposed to be universally
applied, and some letters were just missed, or
whether some sort of selective "profiling" was used to select letters
for censorship. I'm guessing that the huge volume of mail simply meant
that it was impossible to censor every letter, especially given the
manpower shortages that the war had created for the U.K. (On the other
hand, censors weren't always so overworked that they couldn't add
human touches to their work: I have several covers in which the
original return address was partly covered by censor tape; the censor
obviously took time to write the hidden part of the address on the
tape itself.

Your thoughts would be appreciated.

Bob