I just read this really interesting article called, “A4 vs US Letter” by Brian Forte, which explains exactly why we have the two different sizes. I am personally a fan of the Metric system and reading this article has shown me just one more reason that I love it. The A4 paper size was mathematically derived, while the US Letter size was a bit more arbitrary. However, as you can see they are very similar in their size and shape.
For those interested, here is an excerpt from the article:
A ratio of 1:?2 is more than a mathematical oddity. It doesn’t have a nifty name, like the famous Golden Ratio or Golden Mean. It does, however, have a nifty property. Divide a rectangle with sides 1:?2 along the longest side and the smaller rectangle you create has the same aspect ratio. (Markus Kuhn suggested in correspondence we call the ratio the Lichtenberg Ratio, after Professor Georg Christoph Lichtenberg, the German enlightenment figure who first proposed the ratio as a basis for paper formats in 1786.)
Find the full article here. It was quite fascinating to me.