Maybe it was Erik who carved them, he started shipping R/C as a part of his redemption arc. :D Or, if we tone the crackficness down, in a fit of masochistic angst. I doubt that Christine would bother to find the cell and carve their initials there when she went down to bury Erik, though I definitely can imagine her doing it in other time and place. Of course, the likeliest explanation is that the author didn't think things through, but it gives us yet another little detail to make stuff up about.