I've read somewhere, here or elsewhere, about the accidental split. Something gets the most of #2 votes while the frontrunner with the most #1 votes doesn't get enough of them to win the first round so the one with enough #2 votes jumps over it. It's complicated system that should be abolished but AMPAS wants agreeable winners rather than divisive ones so here we are.
Because there's no complete overlap with AMPAS. GG and CC aren't even AMPAS. BAFTA overlaps to a degree but now it's getting more difficult cause jury gets to nominate 2 or 3 not the whole BAFTA body. So that's 12 people's choice. DGA and Director's Branch don't overlap either. It's even worse with SAG-AFTRA where AFTRA is no part of Actors Branch.
That said, every case is different. In Lee case, you can see that GG, BAFTA and DGA enbraced foreign talent but AMPAS went American. Affleck snub for Argo seems to be some kind of split vote with Bigelow (2 political thrillers) that robbed both of a nomination. It's very weird that directors who directed political thrillers got snubbed. Lee won cause nobody really liked Lincoln save DDL performance. Mendes is an interestign case. Parasite win is explicable cause 1917 being one man show was never going to appeal to Acting Branch who are the largest voting block within AMPAS. But Bong upset was out of the left field though Parasite surge overall must have worked its magic.