Those aren't rasp marks, nor were the holes caused by the farrier. Nor do I think it was dampness related, as all the times I've seen the holes in my horses' hooves, it was during the middle of summer, when it's hot and humid but far less muddy than other times of the year.
Kingzy, the mare's "holes" were far less severe than the gelding's, and the've come and gone twice now. As they grow about half an inch down the hoof they just even out and disappear, but they weren't very deep to begin with, either. Hers were also more towards the heel bulb region (for the most part, and like the OP's photo) than the gelding's. Seems like the holes on the white hooves were less noticeable than those on the dark hooves, and I don't think it was just a contrasting color thing, either.