I am no EE so I could be a little wrong. From what I understand, the longer the pipeline, the longer the amount of information takes to travel down. The pent. 4 has something like a 20 stage pipeline which allows for higher amounts of mhz, but has a low ipc. The Athlon has around a 13 stage pipeline, but is not able reach has high mhz. The shorter pipeline allows for a great amount of ipc though.
I'm sure someone will be able to provide more detailed info about this.😉