Column used in free

Dear all,

I am wondering if I can get some ELI5 explanation about the used column of free. I am on a SLES15-SP3 using the latest kernel. Here is the procps version installed:

sles15-sp3:~ # rpm -qa | grep procps
libprocps7-3.3.15-150000.7.25.1.x86_64
procps-3.3.15-150000.7.25.1.x86_64

I am unable to calculate the `used` column based on the man page description:

> used   Used memory (calculated as total - free - buffers - cache)

Here is the output of "/proc/meminfo"​ + "free -k​":

sles15-sp3:~ # cat /proc/meminfo ; free -k
MemTotal:         990456 kB
MemFree:           84672 kB
MemAvailable:     433072 kB
Buffers:             884 kB
Cached:           477372 kB
SwapCached:          236 kB
Active:           485656 kB
Inactive:         281300 kB
Active(anon):     240808 kB
Inactive(anon):    66248 kB
Active(file):     244848 kB
Inactive(file):   215052 kB
Unevictable:        8096 kB
Mlocked:              80 kB
SwapTotal:       1334248 kB
SwapFree:        1333980 kB
Dirty:                36 kB
Writeback:             0 kB
AnonPages:        290560 kB
Mapped:           159496 kB
Shmem:             18324 kB
KReclaimable:      36272 kB
Slab:              82568 kB
SReclaimable:      36272 kB
SUnreclaim:        46296 kB
KernelStack:        4236 kB
PageTables:        15096 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     1829476 kB
Committed_AS:    1503492 kB
VmallocTotal:   34359738367 kB
VmallocUsed:       19020 kB
VmallocChunk:          0 kB
Percpu:              296 kB
HardwareCorrupted:     0 kB
AnonHugePages:    141312 kB
ShmemHugePages:        0 kB
ShmemPmdMapped:        0 kB
CmaTotal:              0 kB
CmaFree:               0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
Hugetlb:               0 kB
DirectMap4k:      126832 kB
DirectMap2M:      921600 kB
DirectMap1G:           0 kB
              total        used        free      shared  buff/cache   available
Mem:         990456      557644       84412       18324      514528      432812
Swap:       1334248         268     1333980
sles15-sp3:~ #

How can one calculate the Used column?

calculated as total - free - buffers - cache

2324704 - 1418652 - 884 - 513644 = 391524
or
2324704 + 1418652 + 884 + 513644 = 4257884

Cache
---
Memory  used  by  the  page  cache  and  slabs  (Cached  and  SReclaimable in /proc/meminfo)
      Cached + SReclaimable = 477372 + 36272 = 513644

Can you someone please let me know? Is it possible to do this?

Thank you,
Samuel