Linux: I have 16GB of RAM, but free -h reports only 13G, and the kernal only using 618MB, why?

I noticed that free -h report 13G of ram available on my Mint Linux 19.3 machine. However, there should be 16GB of ram available. I tried various tests here with the results shown below, to try and account for the discrepancy but can't figure it out can anyone explain?

thinkpad:~$ sudo dmidecode -t memory         
# dmidecode 3.1
Getting SMBIOS data from sysfs.
SMBIOS 3.1.1 present.

Handle 0x0001, DMI type 16, 23 bytes
Physical Memory Array
    Location: System Board Or Motherboard
    Use: System Memory
    Error Correction Type: None
    Maximum Capacity: 64 GB
    Error Information Handle: 0x0000
    Number Of Devices: 2

Handle 0x0008, DMI type 17, 40 bytes
Memory Device
    Array Handle: 0x0001
    Error Information Handle: 0x0007
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 16384 MB
    Form Factor: SODIMM
    Set: None
    Locator: DIMM 0
    Bank Locator: P0 CHANNEL A
    Type: DDR4
    Type Detail: Synchronous Unbuffered (Unregistered)
    Speed: 2400 MT/s
    Manufacturer: Samsung
    Serial Number: 344662CE
    Asset Tag: Not Specified
    Part Number: M471A2K43DB1-CTD    
    Rank: 2
    Configured Clock Speed: 2400 MT/s
    Minimum Voltage: 1.2 V
    Maximum Voltage: 1.2 V
    Configured Voltage: 1.2 V

Handle 0x000B, DMI type 17, 40 bytes
Memory Device
    Array Handle: 0x0001
    Error Information Handle: 0x000A
    Total Width: Unknown
    Data Width: Unknown
    Size: No Module Installed
    Form Factor: Unknown
    Set: None
    Locator: DIMM 0
    Bank Locator: P0 CHANNEL B
    Type: Unknown
    Type Detail: Unknown
    Speed: Unknown
    Manufacturer: Unknown
    Serial Number: Unknown
    Asset Tag: Not Specified
    Part Number: Unknown
    Rank: Unknown
    Configured Clock Speed: Unknown
    Minimum Voltage: Unknown
    Maximum Voltage: Unknown
    Configured Voltage: Unknown

thinkpad:~$ cat /proc/meminfo | grep Slab
Slab:             618032 kB
thinkpad:~$ 
thinkpad:~$ dmesg | grep stolen
thinkpad:~$ dmesg | grep e820
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009efff] usable
[    0.000000] BIOS-e820: [mem 0x000000000009f000-0x000000000009ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x0000000009bfffff] usable
[    0.000000] BIOS-e820: [mem 0x0000000009c00000-0x0000000009d80fff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000009d81000-0x0000000009efffff] usable
[    0.000000] BIOS-e820: [mem 0x0000000009f00000-0x0000000009f0afff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x0000000009f0b000-0x00000000b85a8fff] usable
[    0.000000] BIOS-e820: [mem 0x00000000b85a9000-0x00000000bbd7dfff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000bbd7e000-0x00000000bdd7dfff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x00000000bdd7e000-0x00000000bddfdfff] ACPI data
[    0.000000] BIOS-e820: [mem 0x00000000bddfe000-0x00000000beffffff] usable
[    0.000000] BIOS-e820: [mem 0x00000000bf000000-0x00000000bfffffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fd200000-0x00000000fd2fffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fed80000-0x00000000fed80fff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000100000000-0x00000003beffffff] usable
[    0.000000] e820: update [mem 0xb3d60018-0xb3d6e057] usable ==> usable
[    0.000000] e820: update [mem 0xb3d60018-0xb3d6e057] usable ==> usable
[    0.000000] e820: update [mem 0xb3d52018-0xb3d5f457] usable ==> usable
[    0.000000] e820: update [mem 0xb3d52018-0xb3d5f457] usable ==> usable
[    0.000019] e820: update [mem 0x00000000-0x00000fff] usable ==> reserved
[    0.000020] e820: remove [mem 0x000a0000-0x000fffff] usable
[    0.057910] e820: update [mem 0xb545f000-0xb54effff] usable ==> reserved
[    0.513068] e820: reserve RAM buffer [mem 0x0009f000-0x0009ffff]
[    0.513069] e820: reserve RAM buffer [mem 0x09c00000-0x0bffffff]
[    0.513070] e820: reserve RAM buffer [mem 0x09f00000-0x0bffffff]
[    0.513070] e820: reserve RAM buffer [mem 0xb3d52018-0xb3ffffff]
[    0.513071] e820: reserve RAM buffer [mem 0xb3d60018-0xb3ffffff]
[    0.513071] e820: reserve RAM buffer [mem 0xb545f000-0xb7ffffff]
[    0.513072] e820: reserve RAM buffer [mem 0xb85a9000-0xbbffffff]
[    0.513072] e820: reserve RAM buffer [mem 0xbf000000-0xbfffffff]
[    0.513073] e820: reserve RAM buffer [mem 0x3bf000000-0x3bfffffff]
thinkpad:~$ free -h
              total        used        free      shared  buff/cache   available
Mem:            13G        2.6G        7.0G        172M        4.0G         10G
Swap:          2.0G          0B        2.0G