Why does my iPhone 4S say 12GB even though its a 16GB?

The box says its a 16GB but my phone says its a 12GB why does my phone do that?

Added (1). no I didn't buy it online, my dad gave his old phone to me about a year ago

It might be a fake one. I would find out the authentcity by bringing it to an apple store. Did you buy it online? A lot of places online or not from an Apple store sell fake Iphones.

Because you never truly had a full 16GB worth of space in the first place. Once iOS is installed it takes up roughly 3GB of space, so that automatically drops you down to 13GB, and then you have to factor in the simple fact that formatted memory on iPhone is never a true 1024MB for 1GB but rather 1000MB for 1GB, so you lose almost another 400MB just on that calculation alone meaning you only had something like 15.6GB of actual space to use period for the OS and all other application data.

Storage space for OS.

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