Remember [spoiler:a474f5996f]Hurt's not "the Doctor", even if it's a regeneration of the Time Lord that calls himself the Doctor. That way they keep the same numbering.[/spoiler:a474f5996f] It's all a bit messy anyway, with Tennant half-regenerating in the finale of his 4th season (doesn't that count? I guess it doesn't), and the specifics of regeneration varying wildly between writers. It was never established exactly how it works, and I wouldn't be surprised if their explanation for the Doctor regenerating after 12/13 will be something along the lines of Neil Gaiman's proposed (non-canon) explanation. P.S. : Interestingly, according to Neil Gaiman, a black actor was offered the part of the Doctor at some point, which, if true, is a minor but heartening signal that the execs aren't only looking at white dudes. That might have been before Moffat's time as showrunner, though.