I have been wondering the exact same thing. It is definitely astounding that the common 1920x1080p projector can fit that many pixels into a panel thats approx. 1 inch in size, as that would equal 2200ppi! I know what you mean, Apple's proud 326ppi doesn't seem so impressive anymore! The only explanation I can think of is that most LCD projectors use 3LCD, which are basically 3x 8bit (monochrome) panels, which each have one RGB colour split between them that is then combined to create the full colour image. Perhaps its is a lot easier to create super high ppi panels that are monochrome? Think about it, for an iPhone LCD there needs to be three sub-pixels crammed into every pixel, but for a 3LCD projector there only needs to be one. This is only my speculation (as my many google searches found me nothing), but its the only explanation I could think of. If you find an answer let me know!