Por que o Ubuntu não vê toda a memória que eu instalei?

Question

Como isso pode ser:

$ sudo dmidecode -t memory
[sudo] password for dims:
# dmidecode 2.12
SMBIOS 2.6 present.

Handle 0x0008, DMI type 6, 12 bytes
Memory Module Information
        Socket Designation: DIMM0
        Bank Connections: 0 1
        Current Speed: Unknown
        Type: DIMM SDRAM
        Installed Size: 2048 MB (Double-bank Connection)
        Enabled Size: 2048 MB (Double-bank Connection)
        Error Status: OK

Handle 0x0009, DMI type 6, 12 bytes
Memory Module Information
        Socket Designation: DIMM1
        Bank Connections: 2 3
        Current Speed: Unknown
        Type: DIMM SDRAM
        Installed Size: 4096 MB (Double-bank Connection)
        Enabled Size: 4096 MB (Double-bank Connection)
        Error Status: OK

Handle 0x000A, DMI type 6, 12 bytes
Memory Module Information
        Socket Designation: DIMM2
        Bank Connections: 4 5
        Current Speed: Unknown
        Type: DIMM SDRAM
        Installed Size: 2048 MB (Double-bank Connection)
        Enabled Size: 2048 MB (Double-bank Connection)
        Error Status: OK

Handle 0x000B, DMI type 6, 12 bytes
Memory Module Information
        Socket Designation: DIMM3
        Bank Connections: 6 7
        Current Speed: Unknown
        Type: DIMM SDRAM
        Installed Size: 4096 MB (Double-bank Connection)
        Enabled Size: 4096 MB (Double-bank Connection)
        Error Status: OK

Handle 0x0030, DMI type 16, 15 bytes
Physical Memory Array
        Location: System Board Or Motherboard
        Use: System Memory
        Error Correction Type: None
        Maximum Capacity: 16 GB
        Error Information Handle: Not Provided
        Number Of Devices: 4

Handle 0x0032, DMI type 17, 28 bytes
Memory Device
        Array Handle: 0x0030
        Error Information Handle: Not Provided
        Total Width: 64 bits
        Data Width: 64 bits
        Size: 2048 MB
        Form Factor: DIMM
        Set: None
        Locator: DIMM0
        Bank Locator: BANK0
        Type: DDR
        Type Detail: Synchronous
        Speed: 1333 MHz
        Manufacturer: Manufacturer0
        Serial Number: SerNum0
        Asset Tag: AssetTagNum0
        Part Number: PartNum0
        Rank: Unknown

Handle 0x0034, DMI type 17, 28 bytes
Memory Device
        Array Handle: 0x0030
        Error Information Handle: Not Provided
        Total Width: 64 bits
        Data Width: 64 bits
        Size: 4096 MB
        Form Factor: DIMM
        Set: None
        Locator: DIMM1
        Bank Locator: BANK1
        Type: DDR
        Type Detail: Synchronous
        Speed: 1333 MHz
        Manufacturer: Manufacturer1
        Serial Number: SerNum1
        Asset Tag: AssetTagNum1
        Part Number: PartNum1
        Rank: Unknown

Handle 0x0036, DMI type 17, 28 bytes
Memory Device
        Array Handle: 0x0030
        Error Information Handle: Not Provided
        Total Width: 64 bits
        Data Width: 64 bits
        Size: 2048 MB
        Form Factor: DIMM
        Set: None
        Locator: DIMM2
        Bank Locator: BANK2
        Type: DDR
        Type Detail: Synchronous
        Speed: 1333 MHz
        Manufacturer: Manufacturer2
        Serial Number: SerNum2
        Asset Tag: AssetTagNum2
        Part Number: PartNum2
        Rank: Unknown

Handle 0x0038, DMI type 17, 28 bytes
Memory Device
        Array Handle: 0x0030
        Error Information Handle: Not Provided
        Total Width: 64 bits
        Data Width: 64 bits
        Size: 4096 MB
        Form Factor: DIMM
        Set: None
        Locator: DIMM3
        Bank Locator: BANK3
        Type: DDR
        Type Detail: Synchronous
        Speed: 1333 MHz
        Manufacturer: Manufacturer3
        Serial Number: SerNum3
        Asset Tag: AssetTagNum3
        Part Number: PartNum3
        Rank: Unknown

$ free
             total       used       free     shared    buffers     cached
Mem:       6102336    5605508     496828      31384     769868    3664868
-/+ buffers/cache:    1170772    4931564
Swap:      6282236     103584    6178652

Parece que free só vê 6G , enquanto há 12 instalado. Por quê?

UPDATE

Alguma saída da parte superior de dmesg :

[    0.000000] reg 0, base: 0GB, range: 4GB, type WB
[    0.000000] reg 1, base: 4GB, range: 2GB, type WB
[    0.000000] reg 2, base: 6GB, range: 512MB, type WB
[    0.000000] reg 3, base: 3584MB, range: 512MB, type UC
[    0.000000] total RAM covered: 6144M

UPDATE 2

~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            2.9G   12K  2.9G   1% /dev
tmpfs           596M   21M  576M   4% /run
/dev/sda1       288G   29G  245G  11% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
none            5.0M  4.0K  5.0M   1% /run/lock
none            3.0G  148K  3.0G   1% /run/shm
none            100M   40K  100M   1% /run/user

UPDATE 3

A placa-mãe é Asus P7P55-M , a aparência física dos soquetes de memória é

imagemdomanualestáseguindo

Eu tenho 2 DIMMs 2G cada e 2 DIMMs 4G cada, e estou conectando-os em cada cor por tamanho.