I think it’s just a phrasing issue. Should be: One of them incorrectly used the width of the new line character in its calculations, and so the box that it made

I think it’s just a phrasing issue. Should be: One of them incorrectly used the width of the new line character in its calculations, and so the box that it made was too small [for the text size computed by the other]. I agree it makes it sounds like the box sizing was wrong when it must be the other way around.