Sometimes it's that, but often it's just that concepts don't really map 1-1. Especially humor, often jokes don't really directly translate well even with great translations, but also because there are connotations to certain words or phrases (e.g. allusions to folklore, or just the feeling of a word is different in each language) that could make a direct translation sound bad. Really common example is that you'll often see advertisements directly translated to English like "Let's go traveling!", "Let's eat seafood!" or whatever, but it just sounds lame in English since people don't talk the same way (and ironically sounds cooler to Japanese people because it's in a foreign language).