EMP effects can cover a lot more area. Depends on the height of detonation, and the type of bomb.
Hence it using the power grid to cause the most damage and spread the farthest.
Actually until we experience it 1st hand this is mostly conjecture yes.
There is a very info heavy sticky here in the forum on this.
Just scroll north in the sub-forum we are in.....
https://www.indianagunowners.com/forums/survival-disaster-preparedness/58047-emp-projects.html