The borings are predatory.  If a drilling predator can get at a shell without boring, it does not drill.  For example, moon snails attack razor clams through the gape between the shells and do not drill into them.

There are two possible reasons for the predator to attack a thick part of the shell.  First, it may be the easiest spot to attack, either because of the life habits of the victim or the prey handling techniques of the predator.  Secondly, the thickest part of the shell is generally protecting the meatiest part of the animal, so the extra effort of drilling there may be rewarded with a better meal.

At least some muricids and naticids prefer different parts of the shell to attack, given the same kind of bivalve as victim.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa