Cheers, Fide. Another site to help me while away the hours. The comment by peekay, 'And that is numberwang!', had me doubled up.
I wondered whether there had maybe been some by-elections since 2012 and that the SNP numbers had increased to 438 seats by the end of April. But, nope. Just incompetence, or worse, on the part of the BBC.
ETA: I've just noticed that BBC are saying
So, basically, the BBC can make the results fit any agenda they want.