Puzzlement on memory statistics in Linux

I am working on an embedded Linux system (Kernel-5.10.186) on a SOC platform. Now I am doing long-time testing to collect memory statistics in the system. And I had a shell script to collect memory usage every 5 seconds, and I got following 2 outputs which made me puzzled.

================ No. 37 =================
              total        used        free      shared  buff/cache   available
Mem:          55096       15864       28336         100       10896       36220
Swap:             0           0           0
Active(anon):        148 kB
Inactive(anon):     3736 kB
AnonPages:          3848 kB
  108 dummyx.sh     3292 1056
  112 test_proc        178m 5528
  136 dbus-daemon      4144 1856
  171 adbd              35m 1620
  173 udc_daemon       3292 1288
  174 sh               3292 1392
  175 watchdog         3160 1080
test_proc's RSS: === 5660672
......
================ No. 2584 =================
              total        used        free      shared  buff/cache   available
Mem:          55096       15808       27612         100       11676       36272
Swap:             0           0           0
Active(anon):        132 kB
Inactive(anon):     3256 kB
AnonPages:          3332 kB
  108 dummyx.sh     3292 1056
  112 test_proc        178m 6220
  136 dbus-daemon      4144 1856
  171 adbd              43m 1860
  173 udc_daemon       3292 1288
  174 sh               3292 1392
  175 watchdog         3160 1080
test_proc's RSS: === 6369280

I am watching the memory usage of the system and test_proc, the test_proc RSS is increased from 5660672 to 6369280 by about 700KB, but the used and available memory are almost the same!! Why?