Let $X$ be a compact complex manifold. According to Fulton and Lazarsfeld, a vector bundle $E$ on $X$ is called ample if the Serre line bundle $\mathcal{O}_{\mathbb{P}(E)}(1)$ on the projectivized bundle $\mathbb{P}(E)$ is ample.
This notion should be a generalization of ampleness of line bundles, but I don't quite understand. Assume that $E$ is a line bundle, then its projectivization is isomorphic to $X$. In this case how can we conclude that $\mathcal{O}_{\mathbb{P}(E)}(1)\cong E$?