Late endgame exercise - Fujitsu 8 Game 6

Event: 8th Fujitsu Cup 1990  on Jan van der Steen's web site
Round: 2
Date: 1995-04-03
Black Player: Yu Ch'ang-hyeok
Black Rank: 6 dan
White Player: Nie Weiping
White Rank: 9 dan
Komi: 5.5
Result: B+0.5

Position after move 267  

Captured stones:
B: 13
W: 15

An easy one.

It seems that White wins by 1/2 point. So why does Black win?

Answer