When the Carrera launched, there was no legislation calling for dials to be marked for tritium (which is what that "T" indicates) so early dials don't carry it. But as production really got underway, individual countries began to require the dials to be marked, so we begin to see T dials pretty soon after launch. As the requirement became widespread, most manufacturers mark for tritium globally, rather than making the distinction between territories where it is required and where not. Nonetheless, we still see later watches without Ts occasionally, whether they are for non-legislated markets or part of early trials of non-radioactive lume materials.

If you search on "tritium" in the forum, you should find a number of discussions about it, I think it's a pretty interesting topic myself. If you don't mind me plugging one of my own articles, http://iwmagazine.com/2012/01/13/in-the-dark/ is a good starter read on lume :)