Network interfaces in Windows 7 now cache the current configuration and network information once they make a connection to a network. When a user moves to a second network and then back to the first network, both networks are cached and the connection to the wireless networks are much faster. (It tries to use the same information as before)
Here’s the explanation from TechNet’s Blog
You may want to troubleshoot wireless connections by turning off DHCP Network Hint if you have removed the remembered wireless network, rebooted or worked with your wireless connection. Here’s how to do it.