Anon: I'm confused how this is seki. Surely W will eventually fill up all outside liberties, and then can atari with a dead shape by playing above ?
White captures three stones for 6 points, while black lives with 4 points (assuming black cannot play ko for the whole group). This is 2 points worse than the seki variation, but black gets sente. It will normally be the best variation for both sides.
This black move dies since after 4 black cannot connect his 3 stones and black is left with only one eye around the captured white 2. Black could live with 3 at 4, but they get only two points while losing the corner part of their group.
wdst20 Graphical representation of black living as explained above. Here >you can see how costly this is. Black may live but white gains a lot