How can an approximation ratio be less than 1?
For maximization problems, if the optimal solution is $O^*$ and the worst possible approximate solution is $O$, then the standard way is to consider $O^*/O$ as the approximation ratio. But in some text books, the ratio $O/O^*$ is considered for both minimization and maximization problems and it causes the ratio to be smaller than 1.

You may refer to Approximation Algorithm for more details.