Can you share a Wi-Fi connection using Personal Hotspot?
If you are connecting through the Personal Hotspot on iPhone, it will switch to using the cellular data network that your phone uses.  

This is the nature of how the Personal Hotspot feature is designed. See this article on Apple's website.

So in your case, your laptop was not piggybacking through the wifi connection on iPhone. It's an unfortunate design choice but as far as I know a current limitation. Maybe with a jailbroken device you could get past that but doesn't seem worth it for this case at all.