E3 is an industry-only event. Its purpose is to show off upcoming games and concepts to people in the industry. The early early demos and proof of concepts are like concept drawings of sports cars. Its just a concept. The finished product will always  be different because when they create those concepts they aren't sure about future complications and they dont know exactly what will work and what wont work until they start turning the concept into an actual functining game. They are essentially shooting for the moon and what will or wont work has to be hashed out during the actual development process of the game itself. 
Yet gamers at some point got it into their heads that everything shown at E3 is a public commercial advertisement and what you see is exactly what you should get, and anything different is false advertisement.