Packet loss on first time ping test

After setting up mininet with a normal topology like tree,2. We do the ping test and first time, there is always some packet loss:

mininet@mininet-vm:~$ sudo mn --controller=remote,ip=192.168.60.180 --topo tree,2

mininet> pingall

• • • Ping: testing ping reachability
      h1 -> X h3 h4
      h2 -> h1 h3 h4
      h3 -> h1 h2 X
      h4 -> h1 h2 h3
    • Results: 16% dropped (10/12 received)
      mininet> pingall
    • Ping: testing ping reachability
      h1 -> h2 h3 h4
      h2 -> h1 h3 h4
      h3 -> h1 h2 h4
      h4 -> h1 h2 h3
    • Results: 0% dropped (12/12 received)