drop_caches doesn't empty my cache totally

the situation is simple like this:

ubuntu@ip-172-31-46-92:~$ free -hw              total        used        free      shared     buffers       cache   availableMem:          978Mi       247Mi       526Mi       101Mi       3.0Mi       201Mi       504MiSwap:            0B          0B          0Bubuntu@ip-172-31-46-92:~$ sync; echo 3 | sudo tee /proc/sys/vm/drop_caches 3ubuntu@ip-172-31-46-92:~$ free -hw              total        used        free      shared     buffers       cache   availableMem:          978Mi       246Mi       529Mi       101Mi       1.0Mi       200Mi       506MiSwap:            0B          0B          0Bubuntu@ip-172-31-46-92:~$ 

even though I sudo tee /proc/sys/vm/drop_caches, the cache from free won't go down below 200Mi.. what's wrong with it?