This is the whole point. While in isolation, A generally translates to A' and B to B', it doesn't mean that it can always be used that way.

In translations you'll often get situations where A should be translated to B' in that context (even if in the original language B wouldn't be used there), in order to transfer the concept properly. Literal translations often end up weird, and sometimes completely wrong.

Sorry, but "deceased" is way more formal in English than in French.