> Right, but why does that make letter size confusing?

That's the part I initially quoted; "the paper that the text was printed on wasn’t an A4, it was smaller and not a size I was familiar with. I measured it and found that it’s a US letter size paper at about 21.5cm x 27.9cm"

The author isn't from North America, so they had forgotten the format was different, so they got confused when they assumed it would have been A4 like the rest of the world, but it wasn't.

> the only people in the US who would even notice would be people who commonly do business with other countries or who live near the border

Or, as in the case of the author, they live outside of North American and send/receive letters to/from North America.