Thank you for your suggestion. I immediately looked it up in a dictionary and found "strange" was one of translations of "different", but with a comment "it is colloquial (e.g. http://ejje.weblio.jp/content/different). This may be a reason why 「変わっている」 is not included in the correct answers (to avoid learners' confusion).
Anyway, I learned a lot through this communication and would like to thank you.