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Possibly because of the way my request is routed to their servers?

As FYI, my first try was doing it through my wifi (Comcast) and I just now tried it over my Sprint LTE and got the same result - both ways did show the desktop version. Could be a router or proxy somewhere caching your device info and you'd have to wait to let it age out and try again with iCab.
 
Came up fine for me. Using Puffin Browser on iPhone 6 iOS 8.1

ImageUploadedByTapatalk1416226591.124061.jpg
 
User agent has nothing to do with how they choose to render the site. Your device sends them other important information that it queries for like features that are only available on desktop browsers. What you could do is attempt to disable JavaScript and see if doing that on a desktop user agent browser will do it.

Unfortunately even user agent is more of a simple way of forcing desktop mode.

One way and easy at that is to test for things like html5 or other features which haven't moved to mobile browsers yet.
 
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