I've tried looking around on the current website but I think they've kind of given up on explaining it.

last year's update was the biggest they've ever had because apparently somebody found and sent them a huge amount of foreign lists they didn't have before.

also they are constantly trying to find lists by directors so I assume a few more of those were thrown in (and they post those lists up - they are fascinating) but I assume they got even more lists this year because they claim they are going to update again in December.