The 12 DPI is the hardware limit of the scanner. The software lower limit, as you noticed, is 60 DPI. The Preview uses a 75 DPI scan and the thumbnails are interpolated in the software. The 12 DPI is not used with the Apple software. There may be other third-party software, such as plug-ins, that take advantage of the 12 DPI, but Apple is not aware of any.