Gas problems, not sure if it's a problem or normal?

I was wondering if anyone knew why when I fill my S-10 Blazer up, I'll get 200 miles from being Full till 1/2 tank, then from 1/2 tank to empty I only get about 100 miles or so? Any reasons why?

The top of your tank where the fuel actually sits has straight sides, and the bottom has curved sides holding less fuel.
Most cars are like this.