Finding OH- concentration given the H+ concentration - diverse methods
It is because you assumed the number 14 as a constant while it is actually $-\log(K_\mathrm w)$.
It is $-\log(1 \times 10^{-14}) = 14$ at 25 degrees Celsius but changed to $-\log(9.61 \times10^{-14}) = 13.02$ at 60 degrees Celsius.