Series Eight's unusual episode count (twelve episodes, originally thirteen before an episode was dropped, and the 1985 Christmas Special which was filmed as part of the Series Eight filming block) owes to the fact that it was originally penned as a double-length series, that the BBC could choose to break over two separate series of broadcasts if they so opted to, as they had done with Series Three and Four; and the DVD releases also reflect this option by releasing Series Eight as two halves, although ultimately Series Eight was broadcast as one long series.
Series Eight's episode count (twelve episodes originally thirteen before an episode was dropped, including the 1985 Christmas Special filmed during its production) is because it was initially planned as a double-length series that could be split into two separate broadcast seasons by the BBC, similar to how they handled Series Three and Four; DVD releases also reflect this possibility by releasing Series Eight in two parts. However, Series Eight aired as one continuous season.