(graphic created by me)
American standard "Letter" paper is 8.5 in x 11 in (216 mm x 279mm). A4 standard paper is 8.27 in x 11.69 in (210mm x 297mm).
So I did a bit of research. As it turns out, the "A" sizes that are used in Europe are based on a standard aspect ratio of square root of 2. Additionally, each successive smaller size of paper has it's dimensions constructed as follows: if size A0 has dimensions C x D, where D is smaller, then size A1 will have dimensions D x C/2, A2 will have dimensions min(C/2, D) x max(C/2, D)/2, and so on. Note the pretty picture from Wikipedia:
(graphic taken from Wikipedia)
As always, I think the European system makes more sense, I was just momentarily surprised.