Iphone 4 available memory isless than it should be?

I need urgent help. My iphones available memory is less than it should be. If i add all the apps memory it is 4gb so about 9.7 gb should be left but its showing that 7 gb is left only and sometimes it decreases even if i don't download anything

Most people don't take into account that the operating system (iOS of any version) takes up a good amount of space. I have a 64GB iPhone 5S and it registers in my iTunes as having less than 58GB of memory left. There's nothing wrong with it.