i remember someone on xda giving the following explanation:
(im paraphrasing since i dont have the link right now)
"the only way to test the speed of the connection is by tethering to computer and running speed test. reason being, when using pocket internet explorer on the phone, the phone has to both download the information and render the page, which uses the phones cpu and therefore slows down how fast it can process data and turn that into the page. when u tether and use the computer, the phone is just taking the data and sending it to the computer, so the speed is more accurate"
hope that explains, and ill see if i can dig up the link
edit: found the link...supernatedogg18 was the user who posted it. here it is:
http://forum.xda-developers.com/showthread.php?t=376616
2nd post