Let $0 \rightarrow F \rightarrow G \rightarrow H \rightarrow 0$ be sequence of sheaves of etale sheaves on $X_{et}$ ($X$ is a scheme). To prove they are exact, one way is show that $0 \rightarrow F_{\bar{x}} \rightarrow G_{\bar{x}} \rightarrow H_{\bar{x}} \rightarrow 0$ for every geometric point $\bar{x} \rightarrow X.$ But in many places it is claimed, it is enough to show that sequence is exact for any $U \rightarrow X$ in $X_{et}$, when restricted to $U_{zar}$.
For example, in Proposition 14.1.1 (Page 83) here.
Why is it so?