The Linux kernel and drivers tend to “cache” disk usage with available memory. The memory is reclaimed when a process needs it. This may be what you’re seeing.
From a completely different device that isn’t doing anything right now past running APs and routing, 20 MB are being used as “cache”, as an example
jeff@office:~$ free
total used free shared buff/cache available
Mem: 124500 25356 78104 312 21040 63892
Swap: 0 0 0