What gives with this usb tethering app, thats making by internet blaze through my phone?

Anyways so my cable is disconnected and I have an android 3g which is extremely slow, like 100kb per second so i cant watch youtube, its only good for internet or email on 3g. So I downloaded foxfi and am usb tethering to my computer through my phones signal and my speeds are up to 2gb download speed on the computer. WHY?