In Windows Server 2012 R2, the value of installed memory displayed in Server Manager is smaller than the actual size of the memory installed. This problem occurs when the size of a memory module is equal or larger than 32 GB.
This is a display issue only and doesn't cause any adverse effect in the functionality of the operating system or applications.
This problem occurs because of the way memory is calculated from the System Management BIOS (SMBIOS) memory device information. Server Manager queries the Capacity property in the Win32_PhysicalMemory WMI class to get the system memory size. The Capacity property represents the total capacity of the physical memory in bytes. This value comes from the memory device structure in the SMBIOS. The Extended Size field of the memory device structure is intended to represent memory devices equal or larger than 32 GB which is not used properly to calculate the memory size.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.