What Bob says. Plus Zeiss can decide to make another batch of a discontinued item. For instance, my supplier informed me a few months ago that Zeiss will make a final batch of the Sonnar 85/2 that will be ready in December 2012. Recently the news came that Zeiss have cancelled this plan, not sure why, but probably they didn't get enough demand or have a more priority model to make.
M9 ZM 25/2.8 35/2 50/2 85/2