Replying to Glidecam HD vs XR

So i've been looking at getting a glidecam for a littl while now, and when a b&h catalogue showed up in the mail i immedietly looked at the page with the glidecams. I saw the XR series and the HD series bothig having versions for different weights of cameras and other similar features like the 3-axis gimbal. So why is the HD series so much more expensive than the XR series? The XR series is supposedly for smaller camcorder but what small cam weighs 4-10 lbs? My t3i with my heaviest lens is only 2lbs 7 oz.
someone explain this to me.
TL;DR: whats the difference between the HD series and XR series. Why the price difference