Spiral,
This matter is all to do with the so called 'Correlation problem' i.e. when exactly was 0.0.0.0.0. - the start of the current Mayan World Age - set against the Julian Calendar? Now, the end-date of 2012 Dec 21, is based upon a start date of the current Mayan World Age Cycle (of 13 Baktuns) of 3114 BC Sept 06. This is the generally accepted theory of the day. It is the one that most Mayan Scholars believe to be true. Calleman though is a researcher who, along with quite a number of others over the years, has developed an alternative mapping of the Mayan Long Count onto the modern Western calendar. In a nutshell, that is all there is to it, as such. If you want to see a more extensive list of other alternative theories, see my website page on the Long Count:
Long Count Mayan Calendar
LP