That said, I dunno about the answer to your question. I don't know any technical info about it, but perhaps wifi isn't susceptible to the same type of attenuation as the cellular antenna.
Different Frequency band, different effect. At the risk of starting a whole new debate. When I hold my 3GS in my hand it improves the wifi signal. I see this through a Cydia app called WiFi FoFum it shows the signal strength for wifi in dBm. My wifi signal strength improves from -75 dBm to -67 dBm when held.