I think it's possibly a side-effect of the Dalek Time machine, which the Doctor has stated in some stories to be somewhat crude compared to even the Doctor's old model. So, it could've had a few weird side effects here and there.
It's also possible they had some adventures on their own after returning to the 60s and might have stumbled on something that gave them immortality.
It's interesting, this probably wouldn't have even been brought up if "Mawdryn Undead" had used the visibly older William Russel instead of Nicholas Courtney, as was originally intended.
It's also possible they had some adventures on their own after returning to the 60s and might have stumbled on something that gave them immortality.
It's interesting, this probably wouldn't have even been brought up if "Mawdryn Undead" had used the visibly older William Russel instead of Nicholas Courtney, as was originally intended.
