That's a million dollar question... Is there some specific point of confusion you have?

They differ both in form and in actual us within the sentence. Think of Genitive plural as the form you use in "I have no cars" or "There is a bunch of cars". Words for numbers larger than 4 used to behave as nouns, so the analogy with "a bunch of", "a group of", "a lot of" is better than it seems.