why does an 8 GB iphone say it has 2 GB available when only 4.3 GB has been used?

I have a 8 GB iphone 4 and I was wondering 2 things: 1) It says I've used 4.3 GB but it only has 2 GB available. Where's the other 1.7? And also, when I download a app, why does it take more space than it says its going to? For example, Scarmble with friends says it's 33.1 MB to download but it's taking 63.2 MB right now and I don't have any active games going on right now. How do you delete that history so that the apps don't use as much space?