The different rotor sizes are almost certainly tied to the option and/or trim level of the vehicle. For whatever reason, the catalogs you are looking at aren't discerning the model differences. The smaller rotor will have been used on the base model's wheel and tire diameter (perhaps 15"), while the larger rotor will have been used on upscale models that came from the factory with larger wheels and tires (say, 16, or 17").
I suppose there might be an exception somewhere, but in my experience a 15" wheel can only accommodate a 11" rotor. Maybe 11.25", that kind of thing, but not 12".
Especially since you are looking at a rear wheel fitment, where braking torque is always less than that required for the front axle, I would think you should be able to figure out what to use based on what size wheels are fitted to the vehicle, assuming they are not aftermarket units. Chances are there is a larger front rotor on the front axle than the rear, especially likely with the model with the 12" rear fitment, which is a premium application. The door sticker or manual may also help figure it out.
Also, I would suggest going to TireRack dotcom and see what they recommend. They are extremely accurate in my experience with regard to OEM fitment data, and they definitely do discern the OEM fitment based on option or trim level.