Took this photo from my old iPhone 3GS, so it's a bit unclear, but you should get the idea.... It's for some sort of computer software project, but still matches what could happen during any construction project. And I am sure whoever built a house before would quite agree with this.
One of the point is, sometimes even the customer can't clearly explain what they want. How would you expect the result to be correct after the message been passed over through so many people and documented in different ways.
So make sure you double check and triple check every diagram, every line in the contract, take a lot of pictures, ask a lot of dumb/silly questions - yes it can be a bit embarrassing, but it's really rewarding if this can clarify if every parties involved are talking about the same thing!
Whatever the sales person told you - specially while he/she show you around - write it down and make sure it's correctly and clearly covered in the contract. Whatever promise/agreement you were told/made doesn't count unless it's written down in the contract!