Murray, I haven't inspected one of these, and don't want to get in an argument about it, but the illustration you have posted shows a differential, which would drive both back wheels in the same way as a rear drive car does. Here is an explanation of how it works:
http://en.wikipedia.org/wiki/Differential_(mechanical_device)
It requires a differential rather than one-wheel drive, for the same reason cars have them: if it only had one-wheel drive it would be just about impossible to make turns toward the driven wheel, and the tyre on the driven wheel would wear out rather quickly. Why do they have a differential if they have one-wheel drive? Which wheel would they have chosen? Most of the better ride-ons have differentials, while the very cheap ones usually have a solid rear axle that drives both wheels.
Here is Greenfield's explanation of the advantages of their differential:
"Instructions and Hints
"The Greenfield Evolution mower features a unique and patented new type of differential. This differential offers the operator the option of the full differential action of the rear wheels for improved steering and a tight turning circle, whilst eliminating rear wheel scuffing on fine lawns. Most people will run their Greenfield with the differential operating all the time. This is the preferred method of operation and is accomplished by keeping the handle hooked over the end of the rear axle (see "A" below).
IMPORTANT: Do not attempt to engage or disengage the differential lock from the operator's seat or while the engine is running.
"In some situations, such as extremely wet areas or when working on hills you must lock out the differential action for improved traction. You accomplish this by simply releasing the handle from the end of the rear axle. The lock will automatically engage when released (See A below)."
