Default [Q] cannot auto connect to open wifi even though it is saved

my galaxy s4, running 4.2.2. There are two saved wifi networks, one is open, and the other is wpa security. The wifi switch of my phone is ON. Here is the strange things, when I came in to the range of wpa security wifi, my phone auto connect to the wifi network, everything is ok. When I came in to the open wifi networks area, my phone does not connect to the open wifi, even though it is saved, but I can mannual connect to the wifi.
So, why ? and how can I let my phone connect the saved open wifi auto?