spaceraven:
This one took me a long time to understand. Hopefully my explanation can help anyone who struggled like I did. I thought wE2, bA1, wA2 ko to kill was better than wA1 seki. But it's actually not a seki. White can play A2 to create a bent four in the corner... which leads to ko, similar to the wE2 sequence from earlier. Then why is wA1 correct? The reason is that white can wait until the very end of the game to fight this ko, when white would be sure to win. Black, on the other hand, cannot choose when this ko fight starts. In the wE2 sequence, the ko fight starts immediately, and depending on the board state, white may lose and black may live. By reserving this fight until the end of the game, white can ensure black's death.

Rafar:
after black A1, white can live in seki with E2

Félix:
He can still force ko after P18

MES:
White can force Ko

MES:
SGF