^ One of Doctor Who's greatest strengths, and one of its biggest problems, is that the writers can do anything they damn well please. The set up doesn't have any rules, and any walls erected by one writer can be easily broken by the next. If they're bothering to "respect" continuity at all.
The last time he came back from the abyss it was explained away by the line "the timelords resurrected me". That's it. That's all anyone needed.