iPhone connecting to the captive public Wi-Fi network portal being redirected

I observed a very interesting issue, about iPhone connecting to the captive Wi-Fi network.

Just for example: when I sitting at Starbucks try to connecting the public WiFi, on the WiFi authentication portal. I entry my cell phone number and waiting for the key code send me, while my waiting the key code, I leave the portal and switch to the other Apps. When I got the key code and want to back to the portal to entry it, find the old portal is gone and directing me to a new portal must to repeat the steps again.

On the android device, no such issue, the portal will be there always till i complete the authentication process. 

Any body know the differences?

[Re-Titled by Moderator]