As far as I can tell the RNG doesn't weight anything, so because "monstrous heads" is a basic head option like "Full Helmets", you're as likely to get that as the other, even though the latter is a more common *choice*, let alone the more normal head options.
Random basic head choice, random basic torso choice, random basic leg choice, random basic back choice, then random selections within each of those categories, working down the hierarchy, as far as I can tell.