We have 64 GB of memory, but when I ask "ulimit -a", it shows only 53GB. ulimit -aS gives the same values.

core file size (blocks, -c) 1
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 515567
max locked memory (kbytes, -l) 64
max memory size (kbytes, -m) 56103908
open files (-n) 1024
pipe size (512 bytes, -p) 8
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 8192
cpu time (seconds, -t) unlimited
max user processes (-u) 515567
virtual memory (kbytes, -v) 66227600
file locks (-x) unlimited

How is it possible? What can be the cause of it? How can I ajust it?