I guess a better question might be: why do they even called it DTS Neo "6" ?Because Neo:6 is a matrix decoder that can create a 6th channel, the rear center. Neo:6 is also used to create the rear center channel for the DTS-ES Matrix mode.
It can be used for both 5.1 and 6.1 setups. Check question #7 on this page:
http://www.dtsonline.com/home/faq.shtml
Michael