I just realized something at the end of Ab Aeterno when the MIB is holding the white rock.
As Dogen said to Sayid, every man has a scale. One side represents good, one bad. For the scale in the Cave where the Candidates are written, each side is literally represented by a rock, one white (good) one black (bad).
What I realized is, is that both Jacob and MIB share a scale. The white rock, Jacob. Black rock, MIB. The scale itself is Jacob's metaphorical "bottle and the cork".
When MIB got the white rock at the end of Ab Aeterno, I suppose he was meant to go to the scale and put it on one side, lifting the black rock's scale higher. He got to put his rock down because he found someone who in nature, was good. Richard.
This means "the bottle/cork", or the Island, tipped to the good side. When Jacob died, MIB cast the white stone into the water, causing the black rock to weigh down, indicating the Island has turned to the evil side, to the MIB's side, to his domain.
That's the inside joke. It's like the stock market. One day the Island may be doing "good" (Jacob), the next "bad" (MIB).
This isn't their scale...this is the Island's scale.