Because you're relying on the translator to accurately convey the meaning of words to you, which translators rarely do. Most professional translators and language instructors don't exactly have the best grasp of language themselves. They do some 1 to 1 translation of "monkey" and 猿 as if the connotation, denotation, and etymology are all the same when if you look it up, they're only kinda similar but not exactly.
English Wikipedia:
"Monkey is a common name that may refer to groups or species of mammals, in part, the simians of infraorder Simiiformes."
Japanese Wikipedia:
"日本語におけるサル（猿）とは、通俗的な意味ではサル目（霊長目）のうち、ヒト（古人類を含む）を除いたもののことである。"
Former talks about Simians whereas the latter apparently says (correct me if I'm wrong I don't know much japanese) colloquially all non-human primates. Is the translator going to know whether an orangutan or gorilla qualifies as 猿? Probably not, they're not even going to mention it.
Personally I believe people should learn a language well enough to read Wikipedia( or equivalent), and then continue only with media in the language in question from that point onwards.