why does my iPhone 4s 64 gb says it has 57 gb ??

Formatting mostly but is more down to the misalignment of terminology Mb/Gb and MB/GB

64Gb roughly 59.6GB

64 x (1000/1024)x(1000/1024)x(1000/1024)

So 4Gb is around 3.72GB

250Gb is about 232GB

1Tb is roughly 931GB

or something like that

But the formatting will be proportional to size too as it kind of stores directions to where the data is

AND of course because of the iOS (operational system that the device has installed on it)