It's just a useless label on the cover. PDF/A is nothing but a subset of PDF without proprietary expandability limited to what is considered to “work everywhere

It's just a useless label on the cover. PDF/A is nothing but a subset of PDF without proprietary expandability limited to what is considered to “work everywhere” when dealing with common printed matter. It adds nothing to non-existent error handling rules or parsing strategies. There are 5 different ways for an object to be found undefined/nil, but the specification is silent on whether there's any difference in meaning or handling based on the level it happens. Therefore libraries and tools do what they find most suitable, and anything generated by the numerous easy-to-use sites is potentially not quite the same as originally uploaded. PDF resembles the state of HTML years after HTML4, it barely says what should happen in the best case.