That is an imperfection of the French language (no pun intended) but at the same time such a relief! Imagine if, speaking at the past, you had to choose 1) between passé simple and imparfait, 2) between two other subjonctif equivalents when the sentence implies it. No, you just have to make the first choice and everything that deals with subjunctive is covered by one unique verbal form: the subjonctif imparfait. But that’s become a litterary form, it’s always substituted by subjonctif présent in every type of speech. And there are very few occurences in modern books.

Note that its usage is normal in most other romance languages (Italian, Spanish and Portuguese). Furthermore Spanish and Portuguese also use past simple, and the latter even has a future form of subjonctif which is plainly used in casual speech. French is quite an exception on this point, which makes the things more simple (for once).