There shouldn't be any difference. The fact there are differences is a symptom of a broken procurement process where you get your specs from the person who has purchasing power, or an executive, or really anyone except the people who'll end up using that software.
If you have a saying, you can advocate to include those who will actually use the product.
If you are not forbidden to get in touch with them (believe it or not, some execs will forbid you to include the people you're building for in the conversation), you can try and tighten the feedback loop.
If you're designing to a rigid spec, then it pains me to say it doesn't matter: You will get paid but nobody will use your software. Those are the bitterest dollars.