Why the server shows less total memory compared to the physical memory installed

Issue

The server shows less total memory in free or top compared to the physical memory installed in the server as DIMM modules. For example, the server with 12Gb RAM (3 x 4Gb DIMM modules) shows just 11.5 Gb total memory in the free output:

$ free -b
              total       used        free     shared    buffers     cached
Mem:    12314402816 1381797888 10932604928          0   54702080  566779904
-/+ buffers/cache:   760315904 11554086912
Swap:   17179865088          0 17179865088

$ bc -lq
12314402816/1024^3
11.46868133544921875000