@ Pashuteh Yid
Often the wire does go directly over the top of the pole. Sometimes, however it does not; instead, it goes on the side of the pole or on the crossbar. Therefore, in order to create tzuros ha’pesach, you need to have a lechi that comes directly underneath the wire. An excellent sefer on this is Rabbi Yosef Gavriel Bechhoffer’s “The Contemporary Eruv: Eruvin in Major Metropolitan Areas”.