English to Japanese loses plurals, Japanese to English loses most honoriffics and might need to invent a subject (adding information that shouldn't be there and might be wrong). Different languages also just plain have more words than others with their own nuances, and a round trip translation wouldn't be able to tell which word to choose for the original without a additional context.

Translation is lossy. Good translation minimizes it without sounding awkward, but that doesn't mean some detail wasn't lost.