Chances are what i think is a way off, but you asked.

First, i don't think the carbon deposit is softer. I think the reason it is easier to remove is because of two other reasons.

First, the lead allows for a smoother flow of primer gases into the flash hole, so fewer particles accumulate at the end of the vent liner. The thickness of the carbon build-up is less when there is a lead. A thinner layer of hard carbon makes it easier for the drill to cut through, because there is less material for the drill to remove.

Second, the hard carbon build has a hole through it, virtually the size of the flash hole when there is no lead. When there is a lead, the hole through the carbon is near the diameter of the lead, which is much much larger than the flash hole diameter. This larger opening makes it easier for the drill to penetrate through the carbon, because there is less material for the drill to remove.

Essentially, the lead makes the flash hole less of a blockage, which means less carbon stays at the beginning of the flash hole.

Right or wrong, it makes sense to me. Right or wrong, it even sounds good to me. Hopefully there is some truth in this answer.