In reality the exceptions were mostly a work of fiction. For example, the UK was originally assured that the human rights principles they'd originally proposed as a vague set of aspirations would never be made into law, because they weren't suited to be law. Then the EU did that anyway, so the UK got a "carve out" written into the treaties, and it was reported as such to the public. Then the ECJ ruled that it wasn't allowed to have such a carveout and would have to enforce ECHR and ECJ rulings on human rights anyway.

In other words: people were lied to. There was no carveout, not even when every country signed a treaty that spelled out one clear as day. This is how the EU rolls.