why is my system memory so large?

I recently bought a samsung galaxy A5.. its the 2015 version. My phone has a total of 16GB memory on board but my system memory is 13,77GB and its been like that before i could do anything on it. I figured irs not normal however i bought the exact same phone for my brother and he has the same storage issue. Please tell me, how do i decrease the massive system memory? Please please help me.

A large amount of that (maybe 4-8 GB) is taken up by the operating system.
The rest is either apps that were installed by the carrier or is data.
See if clearing the data cache helps.