I might hazard some informed response on that, as a native speaker of English and student in classroom instruction at various times of German, Russian, Modern Standard Chinese, Cantonese, Literary Chinese, Biblical Hebrew, Attic Greek, Biblical Greek, Taiwanese, Japanese, and Hakka. But what other languages did you have in mind that seemed to be rated as surprisingly easy by contrast with Arabic?
P.S. Being quite the language hobbyist, I have also self-studied a variety of other languages, including several more that are on the list included in the submitted article. Modern Standard Chinese is by far my strongest acquired language, but I would agree that it does not come easily for a native speaker of English--and arguably not for most native speakers of Sinitic languages.
http://news.xinhuanet.com/english/2007-03/07/content_5812838...