I would surmise that different contributors were responsible for different sections of the course, which unfortunately has led to some inconsistency. Part of the purpose of the "Beta" period is for users to flag / report issues such as this. Simply click on "report" and check "my answer should have been accepted." I wish they would provide a way to make suggestions that don't fit the flag report format, that would provide a chance for constructive feedback, but so far I am unaware of that possibility. I invite folks to post on https://www.duolingo.com/comment/24887752
종이 as one word is paper, just keeps messing me up thinking that's a particle, and with the ideograph/character in "종이 세 張을 주세요." it looks like three stretches of paper, literally. In Japanese it gets used to count musical instruments, apparently, probably because the strings are stretched tightly over them?
本 is the Japanese counter for most string instruments; 張 is for umbrellas, parasols, and tents, which makes total sense because those are all fabrics stretched over a frame. The latter, 장, is for pieces of paper and other thin, flat objects because they are all expanses of paper.