What's interesting is that A4 is narrower than the "only in US" standard size of 8.5x11.  Just slightly longer -- A4 is 8.3x11.7.  (Of course, everyone knows that the ratio of long side to short side is the square root of 2.)  

Back in the 1920s, the US government decided it would save money by switching to 8x10.5 for government documents.  Tax forms were in this size for decades.  It was supposed to save money (fewer trees cut down) but of course it cost money because it was a nonstandard size.  Eventually, in the early 80s President Reagan (who also approved the new tax on Social Security) decreed that the federal government would use the 8.5x11 size that the rest of the country was using, and the rest of the world except Canada and Mexico was avoiding.