It may be in part the difference in download speed on wifi vs 3G? The bigger your overall download speed, the larger this effect will be, which will increase the amount of time it takes for the see and digest the content?
Certainly - overall - the trend towards mobile looks unstoppable, so for most businesses, focussing on this metric is the best bet. Desktops are faster, but less convenient for users.
(IMHO) The user is probably able to digest the page faster with a larger desktop view instead of having to scroll thru the phone/tablet to view the information. If the page is mostly text based, I'm sure it can be read faster on the desktop, hence less time on site..
No doubt that "access to the web" these days is going to the mobile venue simply because phones are more common than desktops (pricing for many, convenience for some, all that is available, or simply never had a desktop to start with. With that reality in mind a small screen (on a bus or while waking into walls or a fountain or off a cliff) can only display so much data at a time. Throw in an ad and they have to swipe past that to continue... hence longer times on site with (usually) less value for all concerned... unless a mobile is all they have. And that number is growing by leaps and bounds.