For one, some measure upload speed, and some measure download speed. IIRC, download speed should be faster. Correct me if I'm wrong on that. Another thing to remember, as somebody already pointed out, some have servers close to you, while some may be in other countries.
BTW, I know that my speeds have gone down, because two months ago I could watch espn360, and now I can't. It shows like a frame or two every few seconds. I really hope I get this figured out. I hope Zoomtown doesn't jerk me around like I think they will. Maybe it's just that I'm so used to AOL.