Because of formatting, and what is used for the OS. The amount of physical memory installed is 8 GB, but you lose a few hundred megabytes due to formating, and a gigabyte or two for the OS. This is true of all storage devices. Totally normal. The rest of the free storage is used for apps, games, music etc. Luckily, that phone has an sd card slot so you can add more memory with this.

Hope this helps!