Total RAM in "free" output smaller than real RAM size

I am running Ubuntu 16.04.3 LTS x86_64 on a Lenovo X1 Carbon 3460-4T9 computer having 4 GB of RAM. However, the free command only shows a total of 2610 MB:

vmalep@X1:~$ free -m
              total        used        free      shared  buff/cache   available
Mem:           2610        1525         108         243         976         649

The sudo lshw -c memory command shows all the memory:

*-memory
   description: System Memory
   physical id: 7
   slot: System board or motherboard
   size: 4GiB
 *-bank:0
      description: Chip DDR3 Synchronous 1333 MHz (0,8 ns)
      product: HMT42556MFR6A-G7
      vendor: Hynix/Hyundai
      physical id: 0
      serial: None
      slot: ChannelA
      size: 2GiB
      width: 64 bits
      clock: 1333MHz (0.8ns)
 *-bank:1
      description: Chip DDR3 Synchronous 1333 MHz (0,8 ns)
      product: HMT42556MFR6A-G7
      vendor: Hynix/Hyundai
      physical id: 1
      serial: None
      slot: ChannelB
      size: 2GiB
      width: 64 bits
      clock: 1333MHz (0.8ns)

Output of dmesg:

vmalep@X1:~$ dmesg | grep Memory
[    0.000000] Memory: 2620420K/2807280K available (12300K kernel code, 2481K rwdata, 4004K rodata, 2364K init, 2368K bss, 186860K reserved, 0K cma-reserved)
[    0.092487] x86/mm: Memory block size: 128MB
[    2.791622] [drm] Memory usable by graphics device = 2048M

Kernel in use:

vmalep@X1:~$ uname -r
4.13.0-32-generic

Also, if booting with the live usb (same Ubuntu 16.04.3 LTS version), the 4Gb of RAM are in use...

Any clue?