Not just that, fan subbing tends to be pretty bad. Leaving in honoriffics, just not translating some words, etc
This is precisely why I tend to prefer fansubs. Most professional translations are less precise because they need to target more casual viewers, who would be confused more often than not by untranslated idioms.
Compare to fansubs, which generally target an audience which is more savvy with the language, so leaving certain idioms untranslated allows the viewer to pick up on certain nuances which would otherwise be lost. It's a similar case for honorifics, since they can encode small amounts of additional information that might be otherwise lost or be harder to pick up on.
There's something to be said for going ahead and translating everything since the original language is still there in the audio track, but I think it makes more sense to put everything in the subtitle track, as it may be difficult to follow dialog in both audio and subtitles. Even then, style varies greatly between fansubbers. Some prefer to take the more accessible approach, while others tend to leave idioms untranslated.