The EU has two primary types of legislation, regulations and directives. There are other things such as recommendations etc, but those are usually pretty clear from their name, so I'll exclude those from the explanation. A simple way to think about a regulation is essentially an EU-wide law, it applies the way it was written in Brussels across all of EU. Directives, that's the one in the article, are more like interfaces. They're a guideline of goals to achieve, and every country must implement their own version of the directive.

Why do directives even exist? Because the legal landscape can be widely different between EU countries. Directives give every country flexibility in implementing them in a way that is consistent with the way their laws work, existing precedents, etc. The downside is obviously that the implementations will somewhat differ from country from country.

This means that unless the implementations between countries are fairly consistent, the definition of what working as intended means will vary from country to country.