16 GB Installed RAM and 15.9 GB Total physical memory?

That is correct you would experience the same thing if you partitioned a hard drive as well.

When system boots, it accesses bios and for this it uses ram. Similar to this procedure the rams need a fraction from their storage to get started. That storage is not accessible by other programs, that's why rams show their storage capacity they can use too.. 

that is the total usable memory, it I like that with my rig also...

yes thats correct , a gigabyte = 1,00,000 bytes or 976,5600 kilobytes , yes its correct . your hard drive is measured in gigabytes while your computer runs and measures kilobytes ..simply 1024 = 1 kilobyte.

it is just measurement difference.  forget it

A small amount of the system memory is reserved by hardware for various operations. 

Each memory chip has a small block that is allocated to the system it contains the information about the chip what size is the chip what configuration is the blocks.

This information is permanently flashed to the memory.

It takes up a tiny portion of each stick. So when you install your memory you might have 16 gigs of memory installed but only 15.9 gigs are available to be used the rest of the memory contains the permanently flashed information that makes the memory chips recognized by the system.

This same thing is often seen in hard drives you might have a 500 gig hard drive. But when you install it you will see that only 496 gigs are available!  For the same reason the hard drive has permanently flashed information embedded in it. 

If for some reason that information gets erased or corrupted on either the RAM chips or the hard drive then you run into a big problem.

Neither your hard drive or your RAM chip will get recognized by the BIOS.   Having built up static in your hand when you install the ram can do this.

This is the reason that you are supposed to either wear a wrist strap grounding you to the chassis of the computer.

Or keep your hand touching the metal while you're installing the ram chip. Always unplug it always hold in the power button to discharge any voltage that is still left in the power supply. Before you install ram or a hard drive

Thats normal. Basically ever PC spec is an aproximation or incorrectly converted or has a system reserve.

Some functions of the Operating System Reserve some RAM, Like the Swap File.

One, the motherboard may be using part of RAM for something else and two, the computation of GB is sketchy at best (it's not exactly 1 million). You have 15.9GB. That's very close to 16 which is a lot! You won't miss that .1.