Well duh, it is the same game afterall. No one is expecting a generational change...
And differences were far from minor in the first MS game. The lighting alone saw HUGE improvement. And there were new effects btw, and no it wasn't representative of what we got as a final game. At all.
Well, you must be new to gaming then... It happens, ALOT.
Here's another example for you:
No motion blur, no depth of field, worst lighting, different geometry on objects, worst particles, etc. Also not shown in the picture but the game got overall better texturing in the final version.
And the early demos shown at events was even worst.
You also seem to assume that what we get is the most current build at all times. This is actually very rarely the case.