Obviously the book was translated from US.
I don't remember how bad it was in practice but I remember it made me sad.
(It's okay, I learned immediately how to rectify, uh, squarify, the situation.)
And as I mentioned below - maybe exactly this was in the way from making the best paper plane ever, that could fly all the way to my neighbour? What did I know?
Interestingly, paper A4 wide (210mm?) by letter tall (11") is a thing in the TeX community.
That a4 x 11" document size prints correctly on A4 or letter without scaling or cropping (the margins just change)
My experience is that it is quite difficult to cut the sheet precisely enough if you just use a normal pair of scissors.
The instructions were just there and I was certain it was the best plane ever if I just had the right paper format. And that they failed had nothing to do with my folding skills.