One of the touted benefits of Thor is/was the ability to penetrate to deeply buried structures and this is very true, tungsten being what it is. The other benefit is that it creates a near nuclear level of blast without all that nasty fallout and radiation, which is also true. But you don't actually get the maximum of both at the same time.
The blast occurs form the rapid exchange of kinetic energy between the projectile and the surface. And, like any impact situation, if you elongate the time of the event, you lessen the abruptness of the energy transfer. In this case the more the rod penetrates, the longer it takes and the less abrupt the energy transfer, the less blast that gets generated.
To be clear, by the time the rod has stopped relative to the ground, all the energy is transferred, so you don't actually lessen the amount of transfer, but by achieving a deep penetration, you actually lessen the blast at the surface.
To maximise blast you need something like a Thor "Dumdum" round. A projectile that, upon impact collapses as much as possible and, as instantaneously as possible, transfers all that KE to the ground.
Not that I've given this any thought or anything...
Paul